Effects of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase, on the sleep-waking parameters were studied in rats for 24 hours. Intraperitoneal administration of FMH (100 mg/kg) at 11:30 hr resulted in a longer sleep latency compared with the control values. Hour-to-hour analyses revealed that wakefulness (W) time decreased (from 20:00 to 07:00 hr) and slow wave sleep (SS) time increased (from 19:00 to 06:00 hr) in the night. Paradoxical sleep (PS) time did not parallel the SS changes; it was increased significantly from 07:00 to 11:00 hr in the next morning. The influence of FMH seemed to be divided into direct, immediate action (increase of W) and late, prolonged action (decrease of W), and the results obtained support the histamine arousal hypothesis.