Don't look for the gown Nancy Reagan will be wearing to this inaugural ball to be among the designs in the popular First Ladies' Gown Exhibit at the National Museum of American History . . . even after the ball is over. The museum does not own the second inaugural ball gowns of Pat Nixon, Mamie Eisenhower or Eleanor Roosevelt (though it does own the gowns from Roosevelt's first, third and fourth inaugural balls).

"There is no real procedure about this," said Margaret Klapthor, curator emeritus of the Smithsonian's division of political history, which includes the first ladies exhibit. The Smithsonian has on display the one-shoulder, white gown designed by James Galanos that Mrs. Reagan wore four years ago. In reserve is a black velvet Bill Blass gown made for Mrs. Reagan, "just in case something happens to the other dress," said Klapthor.

The gown displayed is usually the first lady's choice. Bess Truman offered the one she wore to a pre-inaugural party, but the public didn't like it much, so Mrs. Truman substituted another she had worn to a dinner honoring Queen Juliana of the Netherlands. "That's been far more popular," said Klapthor, who tries to stay tuned to the reaction of visitors.

Mrs. Reagan's Galanos gown has been extremely popular with visitors, added Klapthor, who noted that admiring comments are often made about the sparkle of the dress. Visitors often remark as well about Mrs. Reagan's height; she is shorter than former first ladies Rosalynn Carter, Lady Bird Johnson and Betty Ford.

The rug in front of the exhibit wears out about every four years, just as frequently as it does at the Smithsonian's popular displays in the Air and Space Museum and in front of the Hope diamond.