What's described as the "world's best-kept secret" is likely to become the Caribbean's hottest new destination.

All this may appear a trifle odd because the Dominican Republic, of which Santo Domingo is the capital, ranks as the oldest nation in the Western Hemisphere. But never mind. With its discovery by the sybarite, tour groups are certain to follow.

Princess Saroya, ex-wife of the deposed shah of Iran, spent a recent holiday at the island's peachy new resort, Casa de Campo. Happy Rockefeller was there earlier. So were Candice Bergen and one of the Rothschilds.

Casa de Campo, which is 50 miles downcoast from Santo Domingo, is being hailed by Dominicans as the "greatest new resort in the Caribbean." It's not an exaggeration. A development of Gulf & Western, Casa de Campo involves a sprinkling of 143 golf and tennis villas plus 178 hotel rooms across 7,000 acres of verdant land facing the beach.

These are one, two and three-bedroom villas built of native stone and topped with red corrugated roofs. Unfortunately, they're not exactly a steal. tRates during the high season run $175 to $385 a day and $85 to $200 in those off-season months, April to mid-December.

An ordinary hotel room is something else. The low bid in winter is $115 double and summer rates start at $60. And yes, the meals are extra.

But where else in the Caribbean can one get a polo lesson from the nephew of the majarajah of Jodhpur? The price of a horse, mallet, helmet and balls comes to $25, along with a half hour with Jabar Singh. Join a group and you can save $10.

Still, one doesn't have to play the game to enjoy a match. Games go off on schedule every Tuesday, Wednesday and Saturday.

Casa de Campo was created for romantics as well as horse lovers, golfers and the tennis set. There are carriage rides (horse carriages with the fringe on top). Erstwhile Romeos hire them for moonlight jaunts with their Juliets. Others take sunset sails to nearby Catalina Island.

Besides the tepid Caribbean, guests bathe in 10 swimming pools at Casa de Campo. There are 40 other private ones. This isn't counting the six manmade lakes sprinkled across a couple of championship 18-hole golf courses.

The golf courses compare favorably to Pebble Beach. One laid out along the coast is called The Teeth of the Dog, which tells how tough it is. While golf balls disappear with an amazing regularity into the Caribbean, tennis buffs meet the challenge on 15 courts.

Lots at Casa de Campo are on the market for $60,000 to $100,000. You pay extra for the house. Several have spent as much as $500,000 for their digs.

A former cattle ranch, Casa de Campo could easily secede from the nation. It has its own airport and control tower (complete with customs and immigration officers). And there's a 100-man security force. And a fire department. And four restaurants. And a dozen bars.

Besides this, Casa de Campo provides a beach club, a beauty salon, boutiques, a laundry, a car-rental agency and a children's play area. Just to keep the grass trimmed and the jungle cut back takes the combined efforts of 80 gardeners and greenskeepers.

Night owls do a turn at La Jungla, a tropical discotheque. And there's a French country restaurant, Les Canaris, that's set up inside an old plantation house where romance gets high billing.

And just next door -- in the sugar-producing village of La Romana -- Gulf & Western has converted an ancient guest house into a small hotel. One of great charm, it faces the world's largest operating sugar mill. Rooms are furnished with rattan and buildings are topped with flashy red roofs of corrugated metal.

Rates at Hotel Romana range downward from $55 double during the high season to $45 in the April-to-December off season. Seaside suites go for $60 to $35, and for an extra $20 per person the hotel provides both breakfast and dinner.

High on a hill overlooking the entire complex, Gulf & Western is re-creating a 16th-century village where artisans will turn out pottery, paintings and other works. An 18-room inn is opening soon and there will be arcaded shops. Already, a couple of restaurants, La Piazzetta and Cafe del Sol, are doing business.

Wrought-iron lamps glow in the darkness and the voice of a distant river is heard in the village with its adobe walls and shuttered windows, Hollywood's greatest set designers couldn't improve on this new-ancient village with its "crumbling" chapel and cobbled plaza.

Meanwhile, Club Med is preparing its own village a few miles away at Punta Cana. And to the north other resorts are being readied at Puerto Plata with its horse-drawn carriages and ancient thoroughfares.

In the works are hotels, condominiums, shopping plazas, riding schools, and a couple of Robert Trent Jones' golf courses, one of which already is being played.

With the flurry of competition in the hinterlands, resort operators in Santo Domingo are waging a campaign aimed at corraling visitors in the capital. Their main obstacle is a lack of good beaches. They make up for this with a spate of swimming pools of all sizes and shapes.

Without question, the city's finest hotel is the Santo Domingo, a scattering of 220 rooms across 14 landscaped acres facing the Caribbean. Designer Oscar de la Renta's talented touch is visible throughout the hotel with its Dominican marble, wicker, hand-fired tiles, fountain and lattice-work loggias.

Visitors take the plunge in an Olympic-size swimming pool, play tennis on two all-weather courts, dine in the elegant Alcazar and pay anywhere from $45 to $60 for a single room and $50 to $65 for a double.

At the entrance to the new $25-million Plaza Dominica, visitors are greeted by a doorman in pith helmet and khakis. Rooms that face the Caribbean are priced from $34 to $64 single and $38 to $70 double. Add to this the usual 15 percent tax and service charge levied by most Dominican hotels and you come up with the realistic figure.

Sheraton is visible as well ("Learn to dance the Merengue" reads a sign in the lobby). Spanish lessons on Thursday and movies on Sunday. In this low season, singles go for $30 a day and doubles are available for as little as $35. This plus tax and service charge.

Most agree, though, that Santo Domingo's finest little hotel (an inn, really) is the colorful Nicolas de Ovando, its 60 rooms couched behind the walls of a 16th-century townhouse on Calle las Damas. Inside there's the musical ring of a fountain. Wrought-iron chandeliers cast their glow. Bougainvillea hangs in clusters. And for this, one pays from $29 single to $35 double a day.

This oldest country in the Western Hemisphere was discovered by Columbus and ruled as his own private estate for more than 30 years by the late dictator, Generalissimo Rafael Trujillo, the so-called iron man of the Dominican Republic. His assassination in 1961 was followed by several years of civil strife, and eventually free elections.

While the present government is striving sincerely to attract tourists, a nagging question keeps popping up: Why, when one has already paid a $5 arrival tax, is there a $5 departure tax?

Isn't that overdoing it, even when you're trying to catch up?