"President Roosevelt accepted the gift of the West Building in 1941. President Carter accepted the East Building in 1978. It's taken 42 years," President Reagan quipped last night, "but now a Republican has a chance to share in the fun."

The "fun" was a glittering white-tie dinner at the National Gallery of Art's West Building where Reagan received NGA board chairman Paul Mellon's stunning gift of 93 works of art and 75,000 square feet of exhibition space Mellon paid to remodel.

In his remarks about the gallery's founder--Paul Mellon's father, Andrew--the president lauded the philanthropist's personal commitment to the arts. "Our country is blessed with a great patron like Andrew Mellon but also with millions of less wealthy Americans who give what they can." He said that in 1981 the most recent figures showed that Americans had contributed $3.35 billion to cultural institutions and organizations, representing an increase of 3.2 percent over the previous year.

The evening was a chance for Reagan to spotlight the largesse of the private sector that has built and expanded the gallery and its collection through the years. Not one penny of federal funds has ever been spent to acquire works of art for the gallery.

The worlds of politics and art came together over round tables centered with topiary boxwood and heather set up around the statue of Mercury in the gallery's rotunda.

Guests dined on a spinach, salmon and sole mousse, filet of lamb with herbs and a spectacular dessert of spun sugar, meringue and pear ice cream. There were French wines and champagne.

The dinner was the second in the gallery's 42-year history to honor Andrew Mellon, the first only two years ago when Vice President Bush stood in as the ranking administration guest for the then-convalescing President Reagan. Mellon's initial gift of $15 million founded the National Gallery of Art, which opened on March 17, 1941.

Thirty-seven years later, another gift from the Mellons, totaling nearly $100 million, made the East Building possible.

Last night, Paul Mellon traced the history of his father's interest in art, telling the crowd how he traveled in Europe in the late 19th century and "developed a habit of buying a few pictures here and there, though there was no systematic fashion." As he continued to travel and saw great collections in England and the continent, he realized America's need for a national gallery.

"His hope was that this gift of the gallery would serve as a magnet attracting other gifts," said Mellon, describing how the gallery began with 125 paintings and 25 sculptures. "The building was so vast that there turned out to be 24 works of art per acre of gallery space. There was talk of guards not only for security purposes, but to direct visitors to the next painting."

Mellon, with his wife, Bunny, welcomed the president and first lady to the gallery and showed them some of the French paintings they were giving.

Mrs. Reagan was at Mellon's table, seated at his right, and the president was at Bunny Mellon's table.

Among the other guests were 180 gallery trustees, government officials and patrons of the arts, including East Building architect I.M. Pei, Defense Secretary Caspar Weinberger, Chief Justice Warren Burger, Attorney General William French Smith, USIA director Charles Z. Wick, Deputy White House Chief of Staff Michael Deaver and NGA board president John Stevenson.

The silver-haired chief justice swept into the gallery wearing a top hat and maroon velvet-lined cape. "Well, don't you look marvelous," Jean Smith, wife of the attorney general, told Burger. He later confided that he had bought the cape in Madrid.

There were one or two holdouts who showed up in black tie but otherwise there was a white-tie majority. Men who had them wore their decorations; the Knights of the British Empire--which numbered Paul Mellon among its ranks--was one of the more predominant.

From social Washington came Ethel Garrett, Joe and Barbara Allbritton and Evangeline Bruce, whose late husband, David, was first married to Paul Mellon's sister Ailsa. Ailsa Mellon's money was also used in the construction of the East Building.

Mellon said another Andrew Mellon dinner is planned for 1985 and, looking at Reagan, he continued, "We're looking forward to having you with us again in that year--also as president." The crowd burst into applause and a handful of guests rose to their feet--one of them was Chief Justice Burger.