Butoctamide hydrogen succinate (BAHS), related to an organic compound naturally occurring in the central nervous system (CNS), has been shown to increase REM sleep in chronically prepared cats. In the present study, we confirmed that BAHS increases REM sleep in healthy humans, the subjects were six males whose mean age was 21 years and the experiment covered eight consecutive nights. Identical capsules containing either a placebo (linolenic acid) or 600 mg BAHS were administered 1 h prior to recording, which was started at 11 p.m. There was little change in total sleep time, sleep efficiency index, sleep latency, REM sleep latency, or the number of REM sleep periods during the drug- as compared to the baseline periods. There were, however, significant increases in REM sleep and decreases in sleep stages 1 and 2. The night's sleep was divided into three equal portions and analysis of the percentage of sleep stages in each showed that REM sleep markedly increased in the middle third while stages 3 and 4 increased in the last third. A carryover effect of BAHS was recognized during the withdrawal period. The maximum percentage of BAHS-induced REM sleep was 34%. REM density during the drug periods tended to decrease. These results suggest that BAHS may be an efficacious hypnotic in that it increases REM sleep which is suppressed by other clinically used hypnotics.