"What will you do with all those balloons?" a man asked the woman leaving the National Portrait Gallery last night with four brightly colored balloons bobbing in the air behind her, at the end of color-coded ribbons. "I will love them," she said. "What else can you do with a balloon? I will love them and, when they die, I will mourn them."

That was about midnight, after the Great Balloon Raid, which was the final act of a long celebration that had begun seven hours earlier at the White House to mark the opening of a new exhibit, "Champions of American Sport," at the Gallery.

At the White House, Ronald Reagan entertained guests with memories of his own experience as an athlete and a radio sportscaster, back in the days when "baseball was for Caucasian gentlemen only."

"I'm proud that I was one of those in the sports-reporting fraternity who editorialized against that rule," he said. In a lighter vein, he recalled that "If it hadn't been for football, track and swimming, I might not have been able to go to college. We didn't have athletic scholarships in those days; you had to do things like wind the clock in the gym."

In those days, the president said, college sports subsidies were "clean, simple and honorable. Maybe I shouldn't admit this, but I went to a very small school in the Midwest. . . The conference which this school was in had a rule under which you could employ someone as an athletic instructor and still leave him eligible to play. In my senior year, on the starting 11, there were seven physical ed instructors and I was the swimming coach. For eight years, I will confess, I didn't know much about grades like A's and B's scholastically. The eligibility requirement was a C average, and that became my top goal."

Two hours later, at a black-tie dinner for 400 on the top floor of the National Portrait Gallery, Jane Poole of New York City, a nonsmoker, found herself seated (across the table from Sen. Barry Goldwater (R.-Ariz.) and Mrs. Jackie Robinson) between two chain-smokers. "I don't mind it at all," she said. "I rather like it." A moment later, she was explaining that her husband, Harrison, is vice president and treasurer of Philip Morris Inc., a cosponsor of the exhibit with the Miller Brewing Company.

It was an upstairs/downstairs party. Upstairs, where it was exclusive and highly polished, the tone seemed to be set by Philip Morris. On each table, there was a container offering some of that company's more elegant products: Merits, Parliaments and two flavors of Benson & Hedges, while the best known product of Miller had given way to Cedar Ridge Chardonnay 1979, Souverain Cabernet Sauvignon 1976, Domaine Chandos champagne, brandies and liqueurs. The guest list seemed to be composed chiefly of sports luminaries and their relatives, corporate executives of the sponsoring companies, Smithsonian officials and a sprinkling of people from Congress and the administration.

Downstairs, the spirit was more demotic and spontaneous; Miller's beer was available for all, along with popcorn, a hearty buffet, great clusters of balloons floating above the tables in the courtyard, an organist playing like the staff musicians in scores of ballparks, and the exhibit itself, a panorama of the variety of American sports history:

A small bronze of John L. Sullivan knocking out an opponent.

A picture of Johnny Weismuller winning an Olympic swimming medal, next to posters of Johnny Weismuller in Tarzan movies.

A Norman Rockwell painting of Eddie Arcaro weighing in for a race.

A case full of medals won by Jim Thorpe while he was a student at Carlisle.

The Jersey won by Red Grange, blazoned with a big "77" and with brown patches showing prominently on the dark-blue sleeves.

Gloves cast in metal: a bronzed boxing glove that was used by Joe Louis in 1941 (near a painting of his dramatic knockout of Max Schmeling, Hitler's fair-haired boy) and a gold fielder's glove awarded to Willie Mays by the Rawlings Company in 1968.

The 1938 Maseratti that was driven to victory in the Indianapolis 500 in 1948 and '49.

One of the painters featured in the exhibit, Ralph Fasanella, was standing in the lobby downstairs recalling how he worked in filling stations and machine shops until he was "discovered" a few years ago and became self-supporting as a painter. "This is the first time I ever wore a black suit," he said. "I bought it two days ago."

Upstairs, John A. Murphy, board chairman of Miller and a board member of Philip Morris, was telling the black-tie crowd that "we acknowledge the importance of the arts for those who consume our products and in this modest way, we hope to accommodate their demands for culturally fulfilling experiences."