Tonight's reception welcoming His Serene Highness Prince Franz Josef II of Liechtenstein and his fabled family art collection to the Metropolitan Museum of Art was cleverly designed.

After greeting the reigning Prince and Princess Gina and hereditary Prince Hans Adam and Princess Marie, all 375 guests -- Nancy Kissinger, Marietta Tree, U.N. Secretary General Javier Perez de Cueller, William Randolph Hearst Jr., the lot of them -- had to walk through the galleries of Rubenses and Van Dycks, Florentine bronzes, mosaics and porcelains, firearms and a rococo 18th century carriage before they could have a drinkor sit down to dine on roast baby pheasant. It was a daunting route through a collection that dates to the Holy Roman Empire.

''It makes my extravagances look pale and insignificant,'' sighed Malcolm Forbes, whose own family collections focus more on Faberge eggs and toy boats. ''This is awesome.''

''We try to avoid social occasions,'' said Perez de Cuellar, who's braced for several intensely social weeks as the United Nations marks its 40th anniversary. ''But in this case, along with the pleasure of meeting their highnesses, there was the opportunity for the first view of this superb collecion.''

Brooke Astor, whose foundation underwrote the exhibit along with IBM, accepted accolades from William Paley of CBS. ''I'm absolutely thrilled,'' said Astor. ''The royal family has not only lent items from the collection but things off their own walls'' from the castle Vaduz. The show, titled ''Liechtenstein: The Princely Collections,'' opens at the Met at the end of the month.

Douglas and Mary Lee Fairbanks, Sen. Claiborne Pell and his wife Nuala, Douglas and Susan Dillon, Estee Lauder, Samuel and Victoria Newhouse, Sothebys owner Alfred Taubman, several ambassadors, a number of titled Europeans plus 15 serene highnesses of Liechtenstein were among those murmuring respectfully through the galleries. ''It's staggering,'' said socialite Annette Reed. ''We are so luch to have it in New York.''

J. Carter Brown, director of the National Gallery of Art, and his wife Pamela did not entirely agree, of course. The National Gallery was in contention to show the Liechtenstein collection. Brown said he felt pangs that the show's scheduling will conflict with the upcoming show of British treasures in New York.

There was some consolation, however. In the 1960s, when the princely family was less prosperous, the National was able to acquire from it a prized Leonardo. It is, Brown pointed out, ''the only one in this hemisphere. And it's permanent.''