Open for lunch Monday through Friday 11:30 a.m. to 2:30 p.m., for dinner Monday through Saturday 5:30 p.m. to 10 p.m., Sunday brunch 11 a.m. to 2:30 p.m. Reservations, AE, MC, V. Complimentary indoor parking in evening. Prices: At lunch appetizers average $3, main dishes average $5 to $8. At dinner appetizers $2 to $6.50, main dishes $7 to $14.25, average $10 to $12. Full dinner with wine, tax and tip about $25 to $30 a person. Sunday brunch buffet with champagne $15.95. Pre-theater dinner (5:30 to 7 p.m.) $10.95

This is quiz: What is a Metro stop restaurant in Rosslyn that spreads its energies among pre-theater, post-theater, lounge-with lunch, full restaurant, Sunday lunch buffet, gourmet shop and bakery? The answer to all of the above is Tivoli -- and it is uncommonly good.

In the dramatically mirrored third-story dining room, the tables are set in niches, corners and at angles so that diners in the 175 seats have both a sense of privacy and of being right in the middle of the party. The decorations are enormous Chinese vases and sculputres, with flower arrangements to match their scale. And at the center of this dazzling dining room is the wine cellar, with its glass walls and shelves transforming it into a major sculpture.

The wine list is equally dazzling, less for its nine pages, which skim off a bit from all the important wine regions, than for its prices, which hover around two-thirds the prices of most major restaurants. The Clos Reissier 1979 Pouilly-Fuiosse is $16.75, the Tytel Chateauneuf du Pape is $14.25. There is a '74 brunello di montalcino at $18.75, a carvo for $9.75, a vouvray for $10.75; for under $10 you can chose from several respectable California varietals, and for $60 you can get a '75 Dom Perignon.

When the waiters urge a pasta dish called Saracini, ignore the four pastas on the menu -- an agnolotti were doughy with a tasteless filling and a creamy pink tomato sauce that had an off-taste -- and order the pasta they recommend. It is homemade linquine tossed with superbly plump mussels, juicy shrimp, squid rings, sweet (though canned) little clams and a buttery, creamy sauce spiked with dill and scallions. Order it for two and share it as an appetizer or make it your lunch.

Other appetizers not to be missed are notably the smoked salmon, a deep coral fish of supple texture and full smoky taste that is outstandingly good, and the steamed mussels served on the half-shell in a dill butter. The soups -- one day the richest and creamiest clam chowder, another day a full-flavored shrimp bisque sharpened with armagnac -- are elegant. You can also find pate, cold seafoods, baked oysters and snails with spinch among the dinner appetizers. At lunch, there is chopped chiken liver, but ours was strong and sour, quite unpleasant except for being surrounded by wonderful french fried shoestring onions.

Tivoli ordinarily lists as daily specials a soup, an appetizer, a fish, a meat, a pasta and a dessert. In addition, the printed menu includes a few cold entrees -- marinated capon with scallions, terragon and walnut oil and most interesting -- and 14 hot entrees, from paupiette of lemon sole and shrimp poached in with nine nuts to paillard of veal mustard and baked to a crusty brown finish. For fish and meats the kitchen makes good use of fresh herbs; its fish meuniere is New Orleans-style, lightly battered and in a lemony browned butter, topped with fresh tarragon and dill. Salmon, when it is available, is the wisest choice among main dishes. One day it was topped with sorrel leaves that appeared to have been laid on raw, then just wilted by the heat of the lovely light cream sauce -- a unique and delicious dish. Simple dishes such as lamb chops are well executed, the double-thick meat accurately cooked, though mint jelly is a pedestrian accompaniment. Our only disappointment among main dishes was calf's liver with shallots and raspberry vinegar. The liver was thin-cut and cooked as rare as ordered; the raspberry tartness and butteriness were a good if surprising foil for the liver, but the meat itself -- like the chopped liver -- was too strong and showed a slight bitterness.

In all, the food is quite good; sauces are deft and subtle, the ingredients of high quality. Tivoli adds nice fourishes such as slices of brioche with two small pots of butter -- achovy and garlic-parsley -- to start.

What one expects to be the highlights, though -- the elaborate pastries decorating two tiers of a rolling cart -- are disappointing. Those beautiful creations taste dry here, crumbly there, lacking in the butteriness or egginess or whatever might raise them to glory. The best dessert encountered was fresh and plump blueberries topped with superb yellow Devonshire cream thick enough to cling to the berries.

The pastries at Sunday brunch are another matter; not only are the croissants especially good, but the danish are flaky and buttery, the custard-filled slices along worth the trip. There are other reasons as well to try Tivoli for Sunday brunch, which is accompanied by frequent refilling of Franzie champagne. Tivoli readily passes two basic tests of a buffet brunch: conscientious removal of used tableware and good coffee. And while the hot dishes were merely passable, the cold dishes were sufficiently good to compensate. First, there was the fine smoked salmon, available with all the trimmings including whipped cream cheese, bagels and fresh shredded horseradish. There were several seafood salads brightly seasoned, an excellent tarragon chicken with larghe moist chunks of meat, pates of meat and fish that were very good, fresh fruits, cheese, pasta salad, a salad of tiny white beans, a bowl of smoked oysters, and several decent smoked fish, though the herring was disappointing and the curried rice salad had dried out. You are a fine example of the art: good crisp english muffin, very little ham but of fine quality, perfectly poached egg and soft, airy and tart hollandaise.

Given its quality, Tivoli is modestly priced. Given its location, it has a suprising downtown sophistication. Given its beauty, its imagination and its obvious commitment to quality in service and cuisine, it manages to be pleasure garden worthy of the name it has adopted.