Roger and Victoria Sant, the philanthropists who have given blockbuster gifts to local social service and cultural organizations, are donating $10 million to the National Symphony Orchestra.

The pledge is expected to be announced tonight during a concert that marks the official start of the NSO's 75th season. This is the largest single gift in the orchestra's history and the second time the Sants have made a record-breaking gift to the NSO. In 1999 they gave $5 million, and both gifts are directed to an endowment for the NSO music director.

Stephen A. Schwarzman, chairman of the Kennedy Center, said the gift was "adding joy" to the anniversary.

Roger Sant, chairman emeritus of AES Corp., an international power company, said he and his wife were pleased with the development of the orchestra. "The [previous] gift has given stability to the director. It gives the musicians confidence that there is long-term support. We are pleased with the progress of the orchestra and it is just something we felt we wanted to help out," Sant said in a telephone interview.

Sant has been a member of the NSO board since 1996 and is also on the board of regents at the Smithsonian. Victoria Sant is president of the National Gallery of Art and former chairman of the Phillips Collection. The couple recently made a $9 million donation to the Phillips. Since 1998, they have supported efforts to reduce teen pregnancy in Washington.

The NSO plays 175 concerts a year, has an annual budget of $28 million and fills 80 percent of its seats.

Music Director Leonard Slatkin is leaving after the 2008 season. Roger Sant hopes the endowment will help attract quality candidates to replace him. "We have been major supporters of Leonard's. The endowment has affected Leonard's feeling about being here," Sant said.

The gift doubles the money raised privately by the NSO this year. Roger Sant said the impact of the money to an arts group is not easy to measure. "With teen pregnancy, you can measure the drop in teen pregnancy rates. In this case it is much more subjective. Our sense is this community needs a great symphony. We want it to be the real capital that it should be," he said.

Victoria Sant is president of the National Gallery.