Butoctamide hydrogen succinate (BAHS), which is related to an organic compound naturally occurring in CSF, has been demonstrated to increase REM sleep in cats and yound adults. In the present study, BAHS was confirmed also to increase REM sleep in healthy aged subjects. The subjects were six females (68-77 years of age). The experiment covered 8 consecutive nights. Identical capsules containing either a placebo (linoleic acid) or 600 mg BAHS were administered 1 h prior to recording, which was started at 9 PM. BAHS tended to stabilize sleep. The average number and percentage of REM periods increased significantly during the drug nights compared with the baseline nights (P less than 0.05 and P less than 0.02, respectively). The maximum percentage of BAHS-induced REM sleep was approximately 20%. REM sleep did not exceed the upper limit of the physiological range. A carry-over effect of BAHS occurred during the withdrawal nights. During the drug nights, and the average length of REM periods increased in each sleep cycle. The length especially increased significantly in cycle 3 (P less than 0.05). The interruptions of REM sleep decreased in number. The histogram of REM sleep showed that REM sleep increased in the middle and the latter part of the night with two apparent peaks. Though REM sleep increased, REM density decreased. The mechanisms by which BAHS increases REM density decreased. The mechanisms by which BAHS increases REM sleep suggests that BAHS increases serotonin in the brain, and that serotonin increases REM sleep secondarily. BAHS seems to be a unique drug which increases REM sleep, while other clinically used drugs suppress it.