Fight those summer doldrums by attending a concert featuring the Mount Vernon Chamber Orchestra. Saturday's program, directed by Ulysses S. James, will feature Mozart's Horn Concerto No. 4, Haydn's Symphony No. 54 and Bloch's Concerto Grosso for String Orchestra with Piano Obligato.

The concert is to be performed at the Mount Vernon Unitarian Church, 1909 Windmill La. (atop Mason Hill) in Alexandria. It will begin at 8 p.m.

Featured on the French horn will be Roger Whitworth, a former music professor at St. Mary's College in Maryland. Whitworth has performed at the Kennedy Center and the Warner and National theaters and with the Beethoven Nightcappers. He is now a private instructor.

Donations will be accepted at the concert. For further information or directions, call 360-4953. An illustrated slide lecture on the history of Western Hemisphere women painters and sculptors will be presented by Nancy G. Heller, an art historian and free-lance writer, at 7:30 p.m. Wednesday at the Arlington Central Library, 1015 N. Quincy St.

Heller, an Arlington resident who has taught art history and appreciation at the University of Maryland and has lectured at the Smithsonian Institution, is the author of a new book, "Women Artists: An Illustrated History."

The lecture is free. For further information, call Deborah Weilerstein at 284-8181. Catch those Orioles-versus-California Angels next month. The City of Fairfax Recreation Department is sponsoring a bus trip to Baltimore Aug. 30. The bus will leave City Hall, 10455 Armstrong St., at 11:30 a.m.; game time is 2:05.

Cost of the trip and game ticket is $16 per person. Register with the Recreation Department by Aug. 12 by calling 385-7858. For further information, call public information officer Diane Cabe at 385-7855. Community groups planning their fall and winter meeting schedules may reserve The Lyceum Historic Site and Museum for a minimal fee.

The Lyceum, which interprets Alexandria history and culture, is located at 201 S. Washington St. and has an adjoining parking lot. It offers a 200-person-capacity room suitable for meetings, concerts and other functions. It also has a small kitchen and is accessible to the handicapped.

Refreshments, a sound system, audio-visual materials and a piano are available by prior arrangement.

Organizations can qualify for a free presentation by the lecture bureau of the Office of Historic Alexandria, and attendees can visit museum exhibitions and the museum shop. Rental fees help support city programs. For more information, call Jan Killalea at 838-4994.