Someone is stalking the healls of the National Gallery of Art at night, poking a sharp instrument, probably a penknife, into some of the world's great art works.

Although the damage done so far to paintings has been slight -- such as several scratches on the varnish of Renoir's "Dog," or a one-inch incision in Rothko's "Orange and Tan" -- the gallery staff has been shaken.

On March 8, after cuts were discovered ih Matisse's "Masks" and "La Negresse," the FBI was called in by J. Carter Brown, the gallery director.

"It has nothing to do with the public," said Brown, who noted that museums the world over are plagued by vandals. "It's some obviously deranged person who is here after hours."

Since last Dec. 11, when scratches and a puncture first were discovered on four paintings in the "American Nieve" collection, 25 paintings have been damaged.

Paintings have been attacked in both wings of the museum, but mostly in the older West Wing.

Brown said that the art works are checked each day just after the public leaves, then again just before the gallery opens in the morning. The damage had to have been done only when the building was closed to visitors.

As a result, the initial suspects included virtually the entire National Gallery staff of 700 persons -- from curators to guards -- who were in the building at one point or another when the vandalism occurred.

"We're narrowing the suspects," says Brown, who suggests that the FBI has drawn a bead on the culprit. An FBI spokesman declined to give particulars of the investigation.

There is no apparent pattern to the vandalizer's choice of art, His or her taste is eclectic, spanning several centuries of artists.

On Jan. 27, the assailant scratched eight of 15 18th century American paintings located in one room of the West Wing.

"You would suspect the person might be attacking paintings that disturb him. But those paintings couldn't set off anything in anyone," Kay Silberfeld, conservator of paintings at the gallery, said of the eight historical American portraits.

"This makes us feel so vulnerable," Silberfeld said of the staff's reaction. "So far, the damage is so minor. We're all like this," she said crossing her fingers.

Indeed, some of the damge was so minor the paintings were repaired where they hung. Others had to be taken to the painting conservation laboratory.

Yesterday, on a Formica table in the laboratory, work was being done on the penetrated canvas of "Still Life" by Kalf, a 17th century Dutch artist. On the same table, a scratch was being removed from "Capricio" by Canaletto. Both paintins fell victim March 4.

On Jan. 27, a scratch in the varnish in the corner of Stuart's "Lady Liston" was discovered. There followed what Brown called "a mysterious hiatus" until March 4. when five more paintings were scratched.

The FBI checked everyone who had vacations during that time but, Brown said, "it didn't work."

Brown says that probably the most valuable of the damaged paintings is one by the French Baroque artist, Jean-Antoine Watteau (1684-1721), who produced only a few works in his brief life. Watteau's "Sylvia" was scratched.

As for the value of the damaged art work, Brown declined comment. "We never discuss values," he said."That's one of our policies."