WASHINGTON (AP) _ Four Civil War era notes signed by Abraham Lincoln and three celebrated American generals were stolen from a display case in the National Portrait gallery, authorities said.

The notes, tentatively valued at $90,000, were taken between 2 p.m. and 4:30 p.m. Monday, while the gallery in downtown Washington was open to visitors, authorities said. They said the thief pried open the display case.

All four notes pertain to a painting, also on display, of Gen. Ulysses S. Grant on his horse and surrounded by various blue-coated officers, including Gen. George G. Meade, commander of Union forces at Gettysburg, and Gen. George Armstrong Custer.

Lincoln's Sept. 23, 1864, note served to introduce painter Ole Balling to Grant and facilitate the Swedish-born artist's travel in the combat zone. In the other notes, all dated in 1866, the three generals commented on the painting.

In Custer's note, the flamboyant cavalryman later killed by Sioux at Little Big Horn said the painting contained a ''wonderful likeness'' of him, according to a gallery official.

The theft was discovered about 4:30 p.m. by a guard, police said. The gallery's registrar, Suzanne Cox Jenkins, said the notes were mounted under plexiglas in a metal frame bolted shut. She said one corner of the frame had been pried up.

Custer's note measured about 4 by 8 inches, she said. She said the others were on paper measuring 21/2 by 31/2 inches. The $90,000 value placed on the documents was tentative, she said, and might be adjusted after consultations with experts on autographs and antiquities.

The National Portrait Gallery is one of the museums operated by the Smithsonian Institution.