Stone and surprise. When Shakespeare wrote, ''the uncertain glory of an April day,'' he was thinking about inland Croatia. Titanic storms percolate. They flood down over the Velebit mountain range, then bubble away in sunlight. Yugoslavia would seem to have a regular spin-wash cycle. Old women trudge beneath burly five-foot-high stick bundles. You see them in front of the predictable Snak-Bar offering ''hot dog and cheeseburger.'' Hard to say which is an anachronism here. We made our sixth wrong turn and were suddenly pelted by bees. There, beside the road, stood a double 40-foot trailer: one immense hive complex. Between trailer and trailer two shirt-sleeved men sat at lunch. About each head, like electrons around nuclei of some very heavy element, a bee swarm was orbiting. The men waved. Surprisingly. But the most spectacular surprise in Yugoslavia is Plitvice.

We first heard about Plitvice (pronounced PLEET-veet-say) National Park from our Hertz agent at Dubrovnik airport. Sixteen lakes, he told us, each connected to the next by a dramatic waterfall. We were polite, if somewhat condescending, as I recall. Your American is constitutionally suspicious about European natural phenomena. Fine old churches, the decrepit Roman ruin, bathing without a shower curtain - Europe can do that sort of thing with panache. But for nature, well, you vacation in New Mexico. Still, we meant to see more of Yugoslavia than a pretty, yet rather tourist-expurgated, Dalmatian coast. What my wife and I found at Plitvice was - Grand Canyon aside - the most ravishing and accessible natural wonder either of us had ever seen.

We arrived just after 3 P.M. and drove to a parking lot about 1,000 yards beyond our hotel. You can't see Plitvice in one day without blowing your left Reebok out - so two major access points have been set aside. The lower access is trained on stupendous Korana Canyon. And here all heroic devices of language feel wind-broken. Korana is, in the Wordsworthian sense, sublime: Beauty infused by danger. Vertiginous cliff walls with elaborate limestone ornamentation. Rushing water inhabited by mysterious, immense flora. Dark caves surrounded by lush coniferous - deciduous woodland (in autumn it must be even more fetching). And this is the very best aspect of Plitvice: it can be penetrated easily at every significant point. Over the entire five-or six-mile length of Plitvice (the lakes cover more than 450,000 acres), Yugoslav engineers have built a remarkable wooden causeway about six inches above the water - railless, just four feet wide, yet solid - from several hundred thousand narrow planed logs. It is the closest I will ever come to walking on water.

And what radiant water: blue-green, pellucid. For almost five hours we walked upstream - alone at that time of day and year. Into Supljana Cave (a three-legged stool beneath which you could easily insert St. Patrick's transept). Through sunken pastureland now overgrown with enormous horsetail weed. To Sastavci Waterfall, 250 feet high - where a double lake throws itself irascibly down. Sastavci is typical of this system: not one cascade, but 12 or 15, as if the swollen earth were hemorrhaging. And at dusk - spooked somewhat by solitude - we went along a causeway branch into yet another theatrical cavern. Through the basilica-high roof I could see starlight. And so, uncertain whether this was indeed a legitimate egress, and acrophobic as usual, we began to climb bare, eerie 18-inch-wide spiraling stone stairs up. And came out of Korana Canyon just a quarter mile from our car.

Throughout that twilight walk we had been puzzled by tooth-plaquelike fuzz on leaf and stick debris underwater - a sort of gunk sediment you would associate with paper mill discharge in America. Yet the water was gem quality. In fact, as we learned, Plitvice constitutes a rare karst formation. The entire lake system is made of travertine. Subsurface flow dissolves carbonate rock. That chemical salt is then redeposited as surface coating. All the typical cave filigree can be found at Plitvice - but travertine, unlike limestone, accumulates with time-lapse speed. In a real sense Plitvice is alive. This biodynamic process has built the fantastic waterfall barriers between each lake. A fallen branch is swiftly set in travertine. At some spill points the waterfall barrier will grow one centimeter, even two or three, a year.

AS our guidebook made clear: ''The base of an overflow will keep rising until the water shifts to another, lower spill point along the barrier. Such shifts . . . raise the barrier along the whole length.'' A waterfall could reconform daily. The Plitvice cascade chain has been fashioning itself since, oh, Ikhnaton's reign. A one-night stand in geological time.

Next day we crossed Lake Kozjak by electric boat and began circumambulating Upper Plitvice. All the sculptural effects proper to water are here: necklace, flamboyant chute, spring, still tarn, spray. It rushes beneath, extravasates around, bubbles, beats and harangues in each dialect of Yugoslavia. The causeway has been boldly laid out - often right along a precipitous foaming bluff or under torrents through some weird travertine cave. It is, to borrow from Robert Herrick, gorgeous liquefaction everywhere. We attained the upper reach after five hours. Thence a comfortable bus brought us back to square one.

There are three very modern hotels on-site at Plitvice, which is due north of Obrovac, half a day from Split by car. For $58 we stayed overnight at the largest, Jezero, overlooking Lake Kozjak. (There is one $9 per person charge to enter Plitvice National Park itself.) Motels can be found throughout the Lika Valley, but, in summer - we visited in spring - a reservation would probably be advisable. Morale ran high at Jezero. Plitvice is the prime vacation and conference center for Yugoslavs (tennis, sauna and mali - miniature - golf are available). Everyone smiles. Get up to take an aspirin and the bed will be made before you return. English is well spoken.

IF, like me, you're a cave junkie, Postojna is less than one day north by fast Renault. (Yugoslav roads are two lanes, in general, rather unnerving but fun and well-maintained.) From Plitvice to Postojna you'll see sinkhole after sinkhole where the roof of some gigantic cave once fell through - many so large they can hold a two-acre vineyard. Tap this part of Yugoslavia with your knuckle and it'll go ''boink.''

The Postojna caves are enormous but not particularly deep: because of that they can be penetrated for an unusually long distance. You enter the hillside by narrow gauge railway, then travel more than a mile through sumptuous room and Pharaonic gallery. Postojna Station is larger than Grand Central. From there, guides who speak English-through-Serbian chaperone you on a bewitching mile-and-a-half figure-eight walk.

From its main entrance, for several thousand feet, Postojna has been seared black. During World War II the German High Command hid a sizable fuel dump in this natural air raid shelter. But Postojna has at least five access points. In April 1944, a partisan unit insinuated itself through Black Cave, upstream. It blew up 10,000 tons of fuel. Despite this action (and even though Postojna has been heavily explored since 1950 or so), the sensational limestone embellishment is in almost pristine condition - through the entire 19-mile length. Size, by itself, can dumbfound. Great Mountain Room is more than 300 yards high (that's three football fields stacked end zone on end zone). And in Postojna you will encounter a bizarre cocktail frankfurter with feet: Proteus Anguineus. This blind, blunt-nosed salamander is half mammal, half-fish - with lungs for land use and gills for underwater breath. Latest experimental research has shown that Proteus can generate young both by egg production and by live birth.

If you prefer more responsive animals, Lipica is just one half hour away. Here the Lippizaner horse was first bred and trained for Austrian noblemen. At Lipica, stable-paddock tours are available each hour or so in season. The Lippizaner breed was dangerously reduced during World War II, but it has now made a solid comeback. These are powerful, acrobatic animals - a pleasure to watch and to anthropomorphize. Final note: Don't be afraid of Yugoslavia. American tourists are treated with reverent cordiality. And this is your best chance to feel like a person of substance. A hundred dollars will become 900,000 dinars (the average monthly take-home pay in Yugoslavia). A decent three-course meal with wine, tip included, may cost $4. And the restaurant, if it has any pretension to chic, will feature at least one blowup of Marilyn Monroe or Humphrey Bogart. American rock music is ubiquitous. There will be a decorative Red Man tobacco emblem on your table. Children wear wonderfully spurious American T-shirts, run off by some ill-informed logo-forger: my favorite was ''property of the Los Angeles Redskins.'' Borrow a denim jacket from your teen-age son before leaving and everyone will think you were born in Montenegro or Croatia. VISITOR'S GUIDE TO THE REGION Getting There

One can travel to the lake region of Yugoslavia by landing in Dubrovnik and taking the stunning drive north along the Adriatic coast. Or one can land in Ljubljana or Zagreb and drive south. Car rentals are about $150 a week for a Zastava or Yugo, $300 for a Volkswagen Jetta and $500 for a Peugeot 505.

Americans need a visa to enter Yugoslavia. Visas may be obtained by mail from the Yugoslav Consulate (767 Third Avenue, New York, N.Y. 10017; 212-838-2300) or in person at the consulate from 10 A.M. to 1 P.M. No photo or fee is required. Allow two weeks by mail.

More information on visiting Yugoslavia is available from the Yugoslav National Tourist Office (630 Fifth Avenue, Suite 280, New York, N.Y. 10111; 212-757-2801). Plitvice

All hotel prices are for two people in a double room and include breakfast. Rates given are for the high season - July, August and September.

The address for all three hotels within the national park is 48 231 Plitvicka Jezera (Plitvice Lakes). The telephone code for Plitvice is 48. The Hotel Jezero (telephone 76-316 or 76-526) and the Plitvice (76-522) are $55 a night, and the Bellevue (76-344) is about $50.

Among the nearby motels, which all cost $45 a night for two, including breakfast, are the Korana (76-508), about six miles from the lakes, with 180 rooms and three cottages; the Jezerce (76-469 or 76-330), which is nearly two miles from the lakes, with five cottages and 16 rooms, and the Motel Grabovac (77-808), seven and a half miles away, with 36 rooms and 14 bungalows.

One might also consider staying at the Motel Plitvice Maslenica (89-273), which is 75 miles from the lakes on the Adriatic road, a scenic two- to three-hour drive from the lakes. The motel has two restaurants, a pool, sauna and other amenities and costs $44 a night for two, including breakfast. Postojna Cave

The caves are open to visitors all year. Tours are every half hour from 8:30 A.M. to 4 P.M. and at 5 P.M. and 6 P.M. from June 1 to Sept. 1. Off-season, tours are at 9:30 A.M. and 1:30 P.M. Admission is about $5. Lipica

Lipica, six miles from Sezana and next to the Italian border, is the stud farm where Lippizaner horses are bred.

Visitors to the stud farm can attend daily demonstrations of dressage, a kind of exhibition riding. Shows are twice daily, and three times a day in July and August. In addition, they can ride the famed Lippizaner horses. A weeklong dressage package, including advanced lessons, six nights lodging at a local hotel and two meals a day, costs $475; for beginners, $385. Daylong rides cost $10; a course of 10 beginner lessons, $60, and 10 intermediate lessons, $50, according to the Yugoslav National Tourist Board.

From the top, walking just a few inches above the water across Great Cascades in Plitvice National Park; Lake Gradinska, and an arched ceiling in one of the Postojna caves, supported by natural pillars; training masters in the ring at the Lippizaner Stud Farm (Raoul Fornezza);