Four historic and ceremonial objects -- two intricately designed swords and two gold medals -- have been stolen from the Smithsonian Institution after a display case was forced open in the first incident of its kind in 18 months.

Smithsonian officials, who declined to place a monetary value on the items, expressed puzzlement over the loss, which was discovered at 11 a.m. Friday, an hour after the National Museum of American History opened for the day.

"It's obviously an odd assortment of items to steal," said a Smithsonian spokesman.

One of the stolen swords, 35 inches long and encrusted with gold and diamonds, was presented in 1883 by the Viceroy of Peru to Commodore James Biddle.

The other, 38 inches long and ivory handled, was presented by Queen Victoria of England to Rear Adm. Stephen Decatur Trenchard for saving the sinking British bark Adieu on Aug. 14, 1856.

One of the medals was awarded to Navy Lt. John Guest for his role in helping lay the first transatlantic telegraph cable. It was given to Guest by the citizens and chamber of commerce of New York.

The other medal was given by Britain's Royal Geographic Society to Navy Capt. Charles Wilkes in recognition of his 1838-1842 South Pacific explorations that have been credited with the discovery of Antarctica.

Smithsonian spokesman Lawrence E. Taylor noted that whoever took the items left behind a number of others in the display case in the Hall of Military History.

In September 1979, a gold snuff box, valued at $125,000, was stolen from another Smithsonian Museum, the National Collection of Fine Arts. In that theft, the bottom of a display case housing the box was pried open.

In what was said to be the first theft of any kind from the Smithsonian since then, the loss of an ornate silver pen used by John Hay to sign the 1898 treaty of Paris, was discovered last month. The pen apparently disappeared after being taken from its display setting to be photographed for a book. No indication of forcible removal was reported.

Authorities said they know of no connections between the two recent thefts.

The said a thorough search of the museum and the grounds failed to turn up the missing objects, taken from an area routinely patrolled by guards.