This double-blind clinical trial studied 16 acutely agitated psychotic patients with manic or manic-like symptoms who needed rapid tranquilization and were therefore on maintenance treatment. They were randomized to receive intramuscular preparations of clonazepam (1 to 2 mg) or haloperidol (5 to 10 mg) at 0, 0.5, and 1.0 hours. Both medications produced significant reduction of manic symptoms within two hours of initial treatment; however, haloperidol produced beneficial results more rapidly than clonazepam. All patients completed the study, with the exception of one haloperidol-treated patient who developed severe parkinsonism. It was concluded that I.M. clonazepam is an effective, safe, but slower-acting alternative to I.M. haloperidol in the treatment of agitated psychiatric patients in need of rapid tranquilization.