The oldest music having its Washington premiere this week is likely to be also the best: Handel's "L'Allegro, Il Penseroso ed il Moderato," presented tonight as part of the Kennedy Center's Handel Festival.

Jean-Pierre Rampal will leave his flute behind this week when he conducts the National Symphony Orchestra in an unusual and interestingly varied program: Honegger's Symphony No. 2; Liszt's Malediction for piano and strings; Faure''s Ballade for piano and orchestra; and Beethoven's First Symphony. Jean-Yves Thibaudet will be the piano soloist.

Rampal is not the only flutist conducting in Washington this week. James Galway comes to town Saturday night, doubling as soloist and conductor with the National Arts Centre Orchestra of Canada. DANCE

American Ballet Theatre concludes its second week of programs at the Kennedy Center Opera House this afternoon and evening with final performances of Kenneth MacMillan's "Romeo and Juliet." The third and final week, starting Tuesday evening, will be highlighted by the world premiere of a duet, "Walk This Way," by David Parsons (Wednesday night), a salute to Antony Tudor (Thursday night), and on the weekend, four performances of Mikhail Baryshnikov's "The Nutcracker," featuring four different ABT ballerinas as Clara, all of them trained in Washington (Bonnie Moore, Marianna Tcherkassky, Amanda McKerrow and Cheryl Yeager). POP MUSIC

They also rock (and new wave) in Wales: Gene Loves Jezebel, at the Roxy Sunday.

Sound effects specialist and comedian Sam Kinneson, fresh from last week's "Saturday Night Live," is at the Bayou Tuesday. THEATER

For an evening of sublimely stylish nonsense, Noel Coward's 1925 romp, "Hay Fever" (at the Eisenhower Theater). For menace and ominous comedy, Harold Pinter's "The Birthday Party" (at the Studio Theatre). For a classic play, breathtakingly staged, Ibsen's "The Wild Duck" (at Arena's Kreeger Theater).