The effect of flunitrazepam, a new benzodiazepine, on the sleep of insomniac patients under chronic treatment was investigated. Polygraphic recordings showed that this drug decreases not only the activity of the wakefulness system but also the activity of the synchronizing system of slow-wave sleep. The subjective feeling of improved and sounder sleep seems to be related to a decrease of wakefulness pressure and to a decrease of body motoricity but not with the modification of sleep stages themselves. Flunitrazepam appears to possess some regulatory properties on rapid eye movement (REM) sleep, since this stage is enhanced in patients with an initial low amount of REM sleep and decreased in those having a higher initial REM sleep. Flunitrazepam possesses potent and useful hypnogenic properties in man but does not induce physiological sleep.