TIBERIO -- 1915 K St. NW. 452-1915. Open: for lunch Monday through Friday noon to 2:30 p.m.; for dinner Monday through Saturday 6 to 11 p.m. Closed Sunday. Reservations suggested. All major credit cards. No separate non-smoking section. Valet parking at dinner. Prices: lunch appetizers $4.95 to $13, entrees $15 to $19; dinner appetizers $8 to $16, entrees $19 to $35. Full lunch without wine but including tax and tip about $40 to $50 per person.

IL GIARDINO -- 1110 21st St. NW. 223-4555. Open: for lunch Monday through Friday noon to 2:30 p.m.; for dinner Monday through Friday 6 to 10:30 p.m., Saturday 6 to 11 p.m. Closed Sunday. Reservations suggested. All major credit cards. No separate non-smoking section. Valet parking at dinner. Prices: lunch appetizers $4 to $13.90, entrees $11.90 to $18.90; dinner appetizers $5 to $16.90, entrees $14.90 to $25. Full lunch without wine but including tax and tip about $35 to $40 per person.

About a decade ago, Washington leaped from family-priced red- sauce Italian restaurants to expense-account white-sauce Italian restaurants, and only in the past year has some middle ground been established. Moderate prices and high quality typify the newest crop of Italian restaurants: Obelisk, Primi Piatti and Paolo's, for example.

In their wake, the earlier, haughtier expense-account restaurants could have been expected to take a beating. New tax laws trimmed deductibility. New government guidelines left bureaucrats paying their own checks. A diving stock market trimmed the high-income customer base, and heavier competition diluted the attraction of these older, pricier restaurants. At least, these were logical presumptions.

So I decided to see how the high- priced Italian restaurants were taking their hard luck. I returned to two of the exorbitant old faithfuls, Tiberio and Il Giardino, to check the health of the $50 lunch. Although I was able to get a same-day lunch reservation -- a risky bet a few years ago -- I found them both bustling at lunchtime. No signs of humility to be seen, prices not moderated in the least. Even for those not of the three-martini generation, lunch (without wine or liquor) will run $35 to $50 per person for three courses, tax and tip.

MY PERSPECTIVE HAS BEEN AT FAULT. All these years I have been looking at Tiberio as an Italian restaurant, when more accurately it is an Expense Account Restaurant. The telephone manner is suited to secretarial efficiency rather than welcoming hospitality. The service is correct and timely. Like clockwork, a parade of captains, waiters and busboys deliver and clear plates, and occasionally -- three times at my lunch -- somebody asks if you want a glass of wine. The order-taking is orderly. The waiter tells you what "everybody" orders (a combination plate of agnolotti and angel-hair pasta as an appetizer), and you, too, can't resist the pressure. Only later do you find that the agnolotti cost $16 (a whole portion rather than the $9 half portion was automatically served) and the angel-hair $9, so the appetizer selection cost $12.50 a person. But you chalk it up to experience because you have enjoyed a delicious platter of down-soft spinach- filled pasta pillows with cream and parmesan, and an even better angel-hair me'lange of pasta filaments in a tomato sauce light in texture but not in flavor.

Tiberio's menu is long even at lunch, so whether your whim is salmon or scallops, sweetbreads or kidneys, a light meat such as veal or a rich one like duck, it is available. The food doesn't seem particularly Italian. It is more the international standards you expect at first-class hotels. But this time around, it was very, very good.

We tried Dover sole, partly because when Dover sole appears on a menu some other sole often appears on the plate, and partly because it is an expensive imported fish that is rarely very fresh or very good. If a platter of fish at lunch is worth $19.95, this one was. It had the elongated fillets, the firm texture and fragile flavor of Dover sole, and was carefully cooked and lightly crusted with bread crumbs and garlic-parsley butter with a few mushrooms for garnish. This was simple fish at its best. And lamb chops with rosemary were tender, pink, well trimmed and accented by just enough fresh rosemary in a pungent and glossy brown sauce. Both dishes were garnished with herbed zucchini in tomato sauce, one with a boiled potato and another with a potato croquette. To finish, we had American coffee nearly as full flavored as espresso -- a terrific cup of coffee -- and a shared portion of raspberry tart that had a fragile short crust, fresh berries and a slight glazing of jam -- a tart as fine as the coffee.

It was a classic of an expense-account meal: lots of good straightforward food, the excellence of expensive simplicity. The service was all business and a little pushy in encouraging budget excess, and tables were so close that it seemed natural when we were given the check for the next table. But in the hubbub of such a busy dining room, nobody would notice you gasp at a bill of $47 per person (with nothing more luxurious than Perrier to wash it down).

WITH ITS DISCREET ENTRANCE AND profligate space, Il Giardino looks like an expense-account haunt. And it acts like an expense-account restaurant, with waiters as correct and impersonal as loan officers. It is also an extravagantly pretty restaurant -- its floor is tiled in sunny colors, and its pink tablecloths are set with long-stemmed roses. And for once, the money spent on restaurant oil paintings looks like money well spent.

These days, expense-account lunches are expected to be light eating -- the more you pay, the fewer the calories. But forget that at Il Giardino. Here, the richer the food, the better it is likely to be. This kitchen curries favor with its cream sauces. Thin, supple pasta with smoked salmon basks in a delicious cream, its nutty flavor heightened by saute'ed mushrooms. Even thinner capellini are impeccable strands of pasta clinging to a delicate tomato sauce and studded with slivers of prosciutto. Except for excess salt, this was a superb pasta dish, though oddly topped with a slice of melted cheese rather than grated cheese.

If you went right from pasta to dessert, you would leave Il Giardino singing its praises. The dessert cart, with its bowls of brilliant berries and its cakes with waves and swirls of chocolate and cream, is sumptuous enough to divert your gaze from a Tiffany window. White and dark chocolate mousse cakes are light in texture with airy cream fillings and moist spongy cake layers, though as rich and chocolatey as you could want.

But what are those pedestrian main courses doing amid such luxury? Dry, chewy shrimp under prettily browned garlic crumbs? Bouncy, juiceless Dover sole with no flavor except that rubbery undertone the refrigerator sometimes imparts? Veal overpounded to nearly hamburger and topped with limp, bitter peppers that taste straight from a jar rather than from a saute' pan? Luck led us to one delicious main dish, though, a chicken brochette so tender and moist the texture was like swordfish, with the rich flavor of proper grilling. It, too, was garnished with those bitter peppers, but the pale velvety chicken stood above that flaw.

With main dishes came herbed saute'ed zucchini as good as that vegetable can get and waterlogged new potatoes.

At $35 to $40 per person without wine, lunch at Il Giardino may be an attractive way to conduct business, but it is a risky culinary investment. Given the nearly full attendance in the dining room, though, it looks like plenty of investors are willing to take that risk.