"Monkey Business." Paramount. $19.95. Howard Hawks' mordant comedy about rejuvenation, expertly scripted by Ben Hecht, I.A.L. Diamond and Charles Lederer: In it, the wonder youth drug "B-4" is accidentally discovered by an inventive chimp and throws a group of mature and responsible scientists into steadily undeclining years and crazier adventures. The movie shows these scenes of degeneration from Hawks' unruffled, eye-level angle. A great comedy, "Monkey Business" (not to be confused with the Marx Brothers film of the same name) sets its premise like a time bomb, and keeps exploding it at inexorably diminishing intervals. (Marilyn Monroe's role here is subsidiary, but it's a gem--statuesque Miss Laurel, a stenographer to whom her boss barks, "Run out and find somebody to type this.") Information: (213) 468-5000.