The effects of acute intravenous administration of synthetic DSIP, 25 nmoles/kg b.wt, on disturbed human sleep were tested in 6 middle-aged chronic insomniacs. The results were: longer sleep duration and a higher quality of sleep with fewer interruptions; slightly more REM-sleep, but no day-time sedation or other side effects though the sleep enhancing capacity was seen for up to 6 h of night sleep. Sleep-promoting effects occurred only in the second hour after injection, in the first hour a slight arousing effect was indicated. The study corroborates the findings of previous investigations in healthy subjects and shows that DSIP has a normalizing influence on human sleep regulation.