A gold snuff box given by Catherine the Great, 18th Century Empress of Russia, to one of her many court lovers, and currently worth $125,000, has been stolen from the Smithsonian Institution's National Collection of Fine Arts.

The theft, which occurred sometime between Monday and Tuesday morning, is the largest ever at the museum. Because of its value, police and museum officials believe the box was taken by someone with little appreciation for the difficulties posed by trying to sell it for quick cash.

"How are you going to fence a $125,000 snuff box?" asked one police source. "It's out of a national museum. No one would touch it."

But another police source said the thief may intend to melt down the gold box and then sell the gold and the diamonds that are mounted in it separately. "It's obviously much more valuable intact, but it's still valuable in pieces," this source said. The source also said the box may have been stolen for an art collector who would keep it hidden for the pleasure of owning it.

Made of gold and edged with diamonds, the box was housed in a display case in a section of the museum undergoing renovation, museum spokeswoman said yesterday. At the time it was stolen, the burglar alarm system for that portion of the museum was not working because it had been damaged by the ongoing renovation, she said.

The theft was discovered Tuesday morning when a staff member removed a plastic sheet that had covered the display case since Monday to protect it during the renovations. The wooden bottom of the case had been pried open and the alarm wires cut, museum officials said.

The box, about twice the size of a cigarette pack, was Catherine the Great's gift to Prince Gregory Orlov, who helped her seize power from her husband, Peter III, in a coup in 1762.

The enamel panels around the box depict scenes from the coup. They show Catherine leaving the palace on the morning of June 27, 1762; taking the oath that day as empress; and being greeted by nobles and officers.

Its design and construction make it unique, said Thomas W. Bower, assistant registrar of the museum.

The musuem, which shares a building with the National Portrait Gallery at 8th and G streets NW, reported the theft to the police and FBI about 11 a.m. Tuesday but did not inform the press.

"I'm not happy to hear from you," Margery Byers, the museum's public affairs director, said when called by a reporter. She said the museum's decision not to notify the press was prompted by fears that the publicity would lead to further thefts.

Byers said the D.C. police arrived about 4:30 p.m. Tuesday, and the FBI was on the scene by 4 p.m. yesterday.

The box is one of a number of European and Asiatic art objects given to the Smithsonian in 1929 by John Gellatly, a New York art collector. It had been on display since 1976 in a small room housing the Gellatly collection at the National Collection, which primarily exhibits American paintings.

Workmen have been installing smoke detectors throughout the museum and repairing damage allegedly caused by subway construction. Although guards are stationed throughout the museum, they were not assigned full-time to the room where the box was displayed.

"We're heartsick about this," Byers said.

The making of snuff boxes evolved into a high art form in the 18th Century, thanks to the widespread sniffing of snuff by Europe's wealthiest and most aristocratic families. Snuff boxes owned by the fashionable often were made from gold, precious stones, ivory and silver, as goldsmiths, silversmiths and enamelists combined their talents to create what are the museum pieces of today.

The concoction stored in the boxes was powdered tobacco that had been fermented and then scented. Users took a pinch of the powder between their fingers and inhaled it through their nostrils. Though greatly reduced in popularity, snuff still is enjoyed in the traditional way by a few connoisseurs.

The choice of a snuff box as a gift by Catherine the Great was, accordingly in keeping with the fashion of the time.

As for Catherine the Great, her sexual activities while she ruled Russia as empress scandalized not only her own court but all of Europe.

Unhappy in her marriage to Peter III, she took numerous lovers, and eventually one of them, Prince Gregory Orlov, helped her seize the throne. Orlov helped persuade Peter's guard to participate in the coup, which was aided by Peter's own failure to mount resistence and the general low regard with which he was held by the masses.

Catherine ruled Russia for 34 years and Prince Gregory eventually joined the growing ranks of former imperial lovers.