John Hay Whitney was the sort of art collector who hung Picassos in his bedroom.

In his lifetime, he was far too modest to permit the hoopla of a major show. But last night his collection was finally on view at the National Gallery of Art, and everyone from art experts to the family servants came to honor him and his taste.

Paul Mellon, chairman of the NGA board and an old friend, toasted Whitney last night as "ambassador, philanthropist, public servant, soldier, publisher, businessman, connoisseur--of art and of living. He was a friend of human beings and humanistic institutions."

Joseph Alsop, the writer and art collector, said of the 73-painting exhibit: "It's the kind of collection which can only be made with a good eye. Without that eye, the kind of check you can write doesn't matter a damn bit."

"Jock," as he was called by both friends and strangers, was an athlete, heir to a great fortune, ambassador to Great Britain (following in his grandfather's striped pants), chairman of the International Herald Tribune, and trustee on almost every art museum board in the nation.

Last night, many of the other art families of the United States, beginning with Mellon, the benefactor of the National Gallery of Art, and J. Carter Brown, its director, gathered in the East Building to pay tribute to Whitney at the preview of "The John Hay Whitney Collection."

The exhibit opens to the public today and continues through September. It was a fitting setting, because Whitney was on the committee for the East Building. He died at 77, on Feb. 8, 1982.

Mellon noted that Whitney "actively chose these works of art, and the exhibit is a tribute to his initative and personal love of painting."

Mellon, a horseman, noted that Whitney rode to the hounds to hunt foxes and was a six-goal polo player.

Whitney's widow, Betsey Cushing Whitney, and her two daughters, Sara Wilford and Kate Whitney, walked through the exhibit before most of the guests came. John Rewald, who advised them on art for 35 years, credits Betsey Whitney with being moved to great passion for some paintings and great hatred for others.

She was the heroine last night because she finally agreed to this, the first exhibit of the major part of the collection. Whitney himself had always refused to show them all together. Mrs. Whitney has given eight of the 73 paintings in the exhibit to the National Gallery as well as others in the show to Yale University and the Museum of Modern Art in New York.

Whitney, though he'd take the shirt off his back for art, was not fond of taking his favorite paintings off his walls. As Rewald, the exhibit curator, explained earlier, the Whitneys collected for their private delight: "You could be their guest in Long Island or Manhattan and still not see some of their most important paintings."

The great Picassos, which were owned early on by Gertrude Stein, were hung in Whitney's Manhattan bedroom. Rewald noted that on their 500-acre estate in Long Island, they had an indoor swimming pool with sporting prints decorating the walls of the room.

"I do miss having the paintings on my walls at home," Betsey Whitney said.

Laura Haddad, Ellen, Peter and Chris di'Bonaventura and four other grandchildren went through the show, playing a game of trying to remember which room in which house each painting had hung. The evening was a reunion for people who had known Jock Whitney best, including doctors, lawyers, the family's servants and many old friends.

In the great hall of the East Building, under large trees whose twinkling lights made a Milky Way of the dark reaches of the cavernous space, some 220 guests representing the great collectors and collections dined. Pamela Brown, wife of the director, had spent the afternoon arranging the floral centerpieces in carefully casual arrangements.

The dinner guests found the 19th- and 20th-century paintings--all of them in frames which would be handsome even if they were empty--easy to like: beautiful bouquets, well-behaved children, interesting men and women and intricate cubistic constructions. As Mellon put it: "The quality is high."

Whitney's sporting interests were reflected in some of the paintings: seven, including several by Degas, were of horses; 18 paintings, some by Whistler, Eakins and Matisse, were of boats.

The masterpieces, though rare and profound, were so delightful that some guests, previewing the show before the elaborate dinner, felt as though the dessert and champagne had been served first.