Former U.S. Sen. Joseph D. Tydings has filed suit in D.C. Superior Court in an effort to force the sale of the 21-acre Tregaron estate in Cleveland Park to the Washington International School, which occupies the estate under lease.

Tydings is one of six living heirs of the late U.S. Ambassador Joseph E. Davies, who owned Tregaron until his death in 1958. He was joined in filing the suit last week by two other heirs: his mother, Eleanor Davies Ditzen, a Washington socialite, and his sister, Eleanor Tydings Schapiro.

The new litigation does not ensure that the school will be able to buy Tregaron, according to Nicholas A. Adams, attorney for the other three Davies heirs who refused last week to sign a contract of sale to the school for $3.7 million that Tydings and his relatives signed.

These three heirs, who are named as defendants in the Tydings lawsuit, are: Emlen Davies Evers, of Cleveland, one of Davies' daughters; and Jennifer Fitch Moleon and Suzanne Walker Wright of Washington, Davies' granddaughters.

Addams said his clients want the highest price for the estate and are not convinced that the school is offering it. The scholl has been trying to buy Tregaron for several years.

Various real estate developers who have looked at Tregaron rejected the idea of developing it because of its rough terrain and neightborhood opposition to the rezoning necessary to build townhouses there.

Tregaron, with its Georgian mansion and otubuildings, including a replica of a Russian dacha, is at 3100 Macomb St. NW, just east of Connecticut Avenue and adjacent to Rock Creek Park.

The Tydings suit contends that the school, a bilingual elementary and secondary school, has made the "best offer" so far. Tydings could not be reached for comment.