Four Civil War period notes, written and signed by President Lincoln, Gen. Ulysses S. Grant and two other celebrated Union Army commanders, were stolen yesterday by someone who pried open a display case at the National Portrait Gallery in downtown Washington, authorities said.

One of the notes was written by Gen. George Armstrong Custer, who served in the Civil War before being killed by the Sioux at the Little Bighorn. In the note, the flamboyant cavalry leader calls a painting showing him on horseback a "wonderful likeness," according to a gallery official.

The theft of the handwritten notes, valued according to a preliminary estimate at $90,000, apparently occurred between 2 and 4:30 p.m. while the gallery was open to visitors, and was described by a museum official as highly unusual and possibly unprecedented.

All four notes -- those by Lincoln, Grant and Custer, as well as the one written by Gen. George G. Meade, who commanded Union forces at Gettysburg -- had been on display for about eight years and were related to a painting hanging in the gallery.

The painting depicts Grant, who led the Union armies to victory in the war, galloping on horseback in the company of about two dozen of his blue-coated generals, including Meade and Custer.

All four notes were mounted in a display case attached to a railing near the painting, by artist Ole Balling. The earliest of the notes, dated Sept. 23, 1864, was written by Lincoln and served to introduce the painter to Grant, and to facilitate Balling's movements in the combat zone.

In the other three notes, all dated in 1866, the three generals report having seen the completed painting, and comment favorably on it.

Police said the theft was discovered about 4:30 p.m., by a security guard at the gallery, at Eighth and F streets NW. The gallery's registrar, Suzanne Cox Jenkins, said the notes were all mounted under a sheet of plexiglass in a metal frame that was bolted shut. She said the frame had apparently been pried open at one corner.

The theft was discovered about one hour before the 5:30 p.m. closing time, on a day when the number of visitors to the museum was described as somewhat smaller than usual because of the absence of school groups.

Calling the theft highly unusual, Cox said "This is the first time this has happened" in her 15 years at the gallery. The only previous theft she could recall, she said, involved a pair of candlesticks valued at less than $200 and taken from five to 10 years ago.

Other thefts of historical materials have occurred at at least one other Smithsonian Museum in the last few years.

Cox said last night that she had no idea who took the notes or why.

All four of the items were described as relatively brief. The note from Custer was the largest in size and measured about 4 by 8 inches, she said. The others, she said were roughly about 2 1/2 by 3 1/2 inches in size.

Although police provided an estimated value for the documents, Cox said she had not yet had an opportunity to fix their value by consulting with experts in autographs and antiquities.

The documents were particularly important to the Portrait Gallery, she said, because of the difficulty in finding appropriate materials to accompany such works as the painting by the Swedish-born Balling.