More treasures arrive at the National Gallery of Art tomorrow, but they're not from British country houses. Mark Rothko's "Seagram Murals," six large panels, will be installed in the East Building concourse level galleries and can be viewed beginning Wednesday.

The murals were originally commissioned in the late '50s for New York's Seagram Building, but Rothko withdrew from the project when he learned they were to be hung in a restaurant, which he felt was an inappropriate setting for his work. Apparently, Rothko planned to link the paintings in a continuous frieze, although the intended arrangement is not known. According to friends, Rothko wanted the murals to create a closed space that would envelop the viewer, evoking the feeling of Michelangelo's Laurentian Library in Florence.

Rothko completed three sets of panels for the Seagram commission. After his death in 1970, one set was left to the Tate Gallery in London, and the other panels were left to the Mark Rothko Foundation or to his estate. In 1984, the foundation designated the National Gallery as the chief repository of its collection, which includes 285 paintings and works on paper and more than 500 additional reference and study works. The "Seagram Murals" are the first pieces to be transferred to the gallery (most of the rest of the collection is expected by mid-1986).

Two more photographs were added to the gallery's Ansel Adams exhibit this week -- "Trailer House Children" (1944) and "Buddhist Grave Markers and Rainbow," circa 1956. A department of American art staff member said the photographs were not put on display originally because of a lack of suitable exhibition space. However, when it was pointed out that without them Adams' entire museum set was not being represented, gallery officials decided to hang the photos in the entrance to the east room of the exhibition. International Poetry Festival

"It's the first time, that I know of, that a festival is being held in so many languages," says Jose' da Silva, organizer of the International Poetry Festival. The festival, which begins tomorrow, will feature poetry readings in 11 languages, with English translations provided.

Sponsored by the World Bank and the International Poetry Forum, the event celebrates the poetic talent of World Bank staffers, local Washington poets and internationally known poets such as 1985 France Culture Prize-winner Emmanuel Hocquard, 1968 Pulitzer Prize-winner Anthony Hecht and Brazilian Academy of Letters award-winner Marly de Oliveira. Reed Whittemore, E. Ethelbert Miller, Myra Sklarew and Samuel Hazo are a few of the Washington poets reading in the festival.

In 1983, da Silva started World's Words, a multilingual literary magazine for World Bank and International Monetary Fund employes that later was expanded to include works from Washington's international and diplomatic communities. Inspired by its success and feeling that poetry becomes complete when it is read aloud, da Silva, Hazo and Anne Burnham decided to organize this three-day festival. Readings begin at 8 p.m. in the auditorium at the World Bank, G Street between 19th and 20th streets. For information call 477-5315. And the Beat Goes On . . .

The Cultural Alliance is sponsoring a Venetian Carnival, a gala to benefit arts organizations throughout metropolitan Washington. The carnival, Saturday night at Bloomingdale's in Tysons Corner mall, is the first joint project of the alliance and the eight major arts service agencies of metropolitan Washington. Area musical groups will provide entertainment. Tickets are $30 a person; for information call 638-2406.

Washington musicians and singers -- including Shirley Horn, James Horowitz, Frank Maxwell, Marshall Keyes and Gaston Neal -- will perform at a benefit for local musicians. The performance, organized by Horn, will be at 9:30 p.m. Thursday at Banks Restaurant, 4531 Duke St., Alexandria. All proceeds will go to the Musicians Union's "We Care" fund, which helps musicians with severe health problems.

Neo-Expressionist painter Sandro Chia and Howard Fox, curator of contemporary art at the Los Angeles County Museum of Art, will discuss Chia's life and work at 8 p.m. Wednesday at the Hirshhorn Museum and Sculpture Garden. For information, call 357-3030.

The Washington Project for the Arts (WPA) will hold its sixth annual Art Auction at 7:30 p.m. Saturday at the Departmental Auditorium, 12th Street and Constitution Avenue NW. For information and tickets, call 347-4813.