An 18th century lead-cast fountain, valued at between $5,000 and $6,000, was stolen yesterday from the garden at Dumbarton Oaks, D.C. police reported.

Officials said the same fountain was stolen in 1971, but was returned anonymously two weeks later to the grounds of the historic estate.

Police said the fountain, titled "Boy Holding a Dolphin," was repoted missing from the rear garden of Dumbarton Oaks, at 32nd and R streets NW, by a garden superintendent.

The fountain, which is 2 feet tall and weights approximately 300 pounds, is part of a twin set of fountains that were cast in the mid-18th century by an unknown Boyd, a Dumbarton Oaks curator.

Boyd said the fountain was last seen at 5 p.m. Monday when the French and Italian-style garden was closed to the public. She said the fountains were on a terrace behind a 10-foot wall that separates the garden from 32nd Street.

"One part of the matched set is still there," Boyd said, "but it must've taken at least two or three people to lift the twin out of here."

Boyd said the slate-gray fountains were purchased from a Paris art dealer in 1931 by Mr. and Mrs. Robert Woods Bliss, then the owners of the Dumbarton Oaks estate. The 16-acre Georgetown estate now includes the Harvard University Center for Byzantine Studies.

Boyd added that, because of increasing numbers of after-hours trespassers into the estate, barbed wire recently was placed atop the 6-foot wall that encloses the property.

"People here are very upset," Boyd said. "The twins are an important part of the garden."