The Smithsonian Institution's plans to build two underground museums on the edge of the Mall -- one for African art and one for Eastern art -- may be rejected today at a crucial meeting of the National Capital Planning Commission.

The federal planning agency, which has final approval over most federal buildings in Washington, already has expressed concern about the Smithsonian's plans, and the NCPC staff, in a report issued this week, now has taken a strong position against the proposed location for the African museum and has reservations about plans for the proposed Center for Eastern Art.

Despite criticism during the past several months, the Smithsonian has made only minor modifications in its proposal to house the two museums in one huge $50 million underground building behind the original Smithsonian Castle. The Smithsonian proposes to raise $25 million itself and would ask Congress to provide the other $25 million.

The plans call for two above-ground pavilions to serve as entrances and small exhibition areas, but 96 percent of the 460,000-square foot building would be below ground. The four-acre site -- now a Victorian garden and an employe's parking lot -- is bordered by Independence Avenue and the Mall's three oldest buildings -- the castle, built in 1855; the Arts and Industries Building, built in 1881; and the Freer Gallery of Art, built in 1923.

Both the joint Committee on Landmarks and the NCPC staff have not objected to the concept for an underground Center for Eastern Art, which would be an extension of the Freer and also would provide underground office space for other Smithsonian activities and an underground parking lot for employes and the public. A small pagoda-like pavilion above ground also has not been opposed in principle, but critics say the pavilion called for in the plans -- which would be 39 feet high, 62 feet wide and 92 feet long -- would be too large.

The major opposition, however, has come since the Smithsonian decided to enlarge the underground building and to add a second above-ground pavilion to house its newly acquired Museum of African Art. The once-private museum, established in 1964 in the Frederick Douglass house on Capital Hill, was incorporated into the Smithsonain in 1979. The collection, now being expanded, already has outgrown its present location.

The Smithsonian wants to put the African museum on the Mall to give it stature and increased visibility. But the NCPC staff, the Joint Committee on Landmarks and the prestigious Committee of 100 -- Washington's first unofficial planning agency, founded in 1923 -- argue that "burying" the African museum in a relatively untraveled part of the Mall will not accomplish those goals.

All three groups recommended that the African museum be put downtown near the Gallery Place Metro subway station, where the Smithsonian already has two art museums -- the National Portrait Gallery and the National Museum of American Art (formerly the National Collection of Fine Arts). Numerous private art galleries are also in the area, and the city soon will open the old Lansburgh's department's store as an arts center. Several historic buildings, such as the 19th century Le-Droit Building, have been mentioned as possible sites for a new African museum.

The NCPC staff also has suggested that a new building be constructed for the African museum on the "last museum site on the Mall," the now-grassy triangular block at the foot of the Capitol, bounded by Maryland and Independence avenues and Third and Fourth streets NW.

The NCPC staff report insists that the underground site proposed by the Smithsonian "is 'on the Mall' only in the sense of a very liberal interpretation of that term," since it would be buried behind a row of Mall buildings.

Another objection raised by the NCPC staff is that the Smithsonian's plans call for construction of two brick walls along Independence Avenue that would be 7 feet high and 157 feet long. The staff says this would create too much of a barrier between the avenue and the Mall.