The effects of L-5HTP 200 mg on the EEG sleep pattern were investigated in normal subjects. When L-5HTP 200 mg was orally administered, only % S1 significantly decreased in comparison with the baseline. There was no change in any sleep stage on the first recovery night. On the second recovery night, % S1 significantly decreased, while % SR increased significantly. SWS did not show any change. The effects of L-5HTP 200 mg were investigated on the first recovery night after one night of sleep deprivation. SWS significantly increased on the first recovery night when a placebo was given, while this rebound increase of SWS disappeared and % S2 significantly increased on the first recovery night when L-5HTP was given. % SR significantly increased on the next night after the L-5HTP night.