Olney Theatre has scheduled the most ambitious season in its 34-year history -- a six-show series that will open May 20 with Ira Levin's thriller "Deathtrap" and run 24 weeks.

"There has always been a demand for good professional theater in the summer," said Bill Graham Jr., managing director of the white-frame playhouse, yesterday. "But we have found that there is also a demand in the spring and the fall."

Rising attendance -- 37,000 (1983), 45,000 (1984) and 70,000 (1985) -- has prompted the current expansion, he said. Olney ended up playing 24 weeks last year, but only after its hit production of "The Foreigner" was brought back for a four-week postseason engagement.

This year, Graham said, Olney has opted for an extended season from the start in an effort to "become a more important part of the full-time Baltimore-Washington theater community."

"Deathtrap," which will be staged by Charlie Lang, will be followed by the Howard Ashman/Alan Menken musical "Little Shop of Horrors" (opening June 17); Harold Pinter's "Betrayal" (July 15); Robert Bolt's "A Man for All Seasons," starring Philip Bosco as Sir Thomas More (Aug. 12); Willy Russell's "Educating Rita" (Sept. 9); and the Stephen Sondheim musical "A Little Night Music" (Oct. 7). Each production will play four weeks.

Olney was recently awarded a $15,000 grant by the Eugene and Agnes E. Meyer Foundation, which will be used to increase the rehearsal period for each show from two to three weeks.

Although Olney has long enjoyed the reputation of being the preeminent summer playhouse in the area and was officially designated the State Summer Theatre of Maryland by the Maryland General Assembly in 1978, Graham views the upcoming season as a turning point. "We're no longer just a summer theater," he said. "Summer is a state of mind."