The rich are different from us . . . except when it comes to free food.

One recent sweltering night 1,200 of them swarmed over the unfinished interior of Market House on M Street at a party honoring the Georgetowners who helped make the restoration of that former market possible. That's 1,200 out of a possible 700 invitation.

The food was provided by some of the 19 vendors who will open for business on July 23. The biggest traffic jam was between Landsdowne's hot sauteed shrimp counter and Lump Que's hickory-smoked barbecued ribs, chicken and potato salad and coleslaw. Even "excuse me" wouldn't budge some of those who were waiting until more of the 100 pounds of shrimp were sauteed. That made it difficult to get through the aisle to the back of the market where Cannon's was dispensing freshly shucked oysters and clams.

By 8 o'clock, an hour and a half after the party started and an hour before it was supposed to be over, Cannon's had run out. Lump Que's had no chicken. The Peking Duck had closed up shop entirely and Innsbruck Strudel and Ice Cream was without strudel; only its Bassett's ice cream was left. The nuts at Barry's candies, dried fruit and nuts had gone limp.

Those who arrived late, such as Bess and Tyler Abell had to do without. Early arrivals -- Councilwoman Polly Shackleton, Ann and Wyant Vanderpool and Kay Halle, one of the honorees -- had been among those who swept the platters clean. Some had systematically worked their way back from Hudson Brothers crudites with dips, through Le Palais' croissants, pains au chocolat and chocolate cake to Zeppole's calzone and pizza. Engrossed in conversation, and dripping barbecue sauce, few of them noticed that the "dessert" had come between the "first course" and "main course."

It's unlikely that the vendors in this 20th-century restoration will bear much relationship to the farmers who hawked their wares in 1795 when the first market opened. The building itself dates from 1865. The farmers will be located in stalls outside on Saturdays.

But you will be able to get fresh pasta at Pronto restaurant every day, on the lower level. And the calzone and pizza on the mezzanine, along with other seating for the carry-out food you buy on the main level.

Altogether there will be 19 concessions (if the sushi-tempura bar people sign a lease) from which to buy food to eat there, carry it out and eat at home, carry it out and cook at home.

If all the people who ate their way through Market House last week come back with their friends everyone will make the rent, pegged at $42 to over $100 a square foot.

If your favorite oyster recipe is just waiting to be a prize winner, enter it in St. Mary's County Oyster Cook-Off before Aug. 15. If you are a finalist it will give you an excuse to drive down to Leonardtown, Md., and enjoy the annual oyster festival Oct. 18 and 19.

Should your oysters turn up in the very best dish, you will also get $500 for your trouble. And there will be additional cash prizes in each of the four categories: hors d'oeuvres, soups and stews, main dishes and stuffings.

If you'd like an entry blank write to: Oyster Cook-Off, P.O. Box 441, Leonardtown, Md. 20650, Att: Mrs. Hope Martin.

Counting calories? The Capitol Hilton's Coffee Shop will help you with its Fitness First menu, which features a picture of marathon runner Frank Shorter.

The menu offers three different lunch and dinner entrees each day complete with calorie counts. If you insist on a first course or dessert, you're on your own. But these entrees frequently have well over 400 calories so they ought to be enough for most people.

The menus were put together with the aid of registered dietitians and Castle & Cooke Foods so, needless to say, there's plenty of pineapple and tuna. The hotel will even give you a pamphlet containing some of the dishes served as part of Fitness First.

Non-smokers who feel strongly about the smoke of others might find the Guide to Smoke-Free Dining of some help. Most of the places listed have dining areas designated for non-smokers. Some restaurants, noted with an asterisk, prohibit smoking completely. As the author, Michael Horowitz, notes, most restaurants which are entirely smoke-free serve natural and/or vegetarian cuisine.

There are 5,000 restaurant listings for the United States and Canada. Copies of the guide are available for $5.75, including postage from: American Lung Association of Western New York, 766 Ellicott St., Buffalo, N.Y. 14203.