Sodium gamma-hydroxybutyrate (GHB) is a remarkably safe and nontoxic hypnotic agent which is reported to be free of addicting properties. It is also a normal metabolite of the mammalian nervous system. We examined its effects on the sleep-EEG of eight patients with histories of impaired sleep, as a prelude to a more detailed study of its clinical potential. Sleep induced with GHB was indistinguishable subjectively from natural sleep as well as by behavioral and electroencephalographic criteria. Unlike most synthetic hypnotics, GHB increased delta sleep and did not suppress REM sleep. It shortened the REM sleep latency and shifted REM sleep into the first third of the night. On one occasion it induced a sleep onset REM period which was experienced as an attack of sleep paralysis. Withdrawal was simple; there was no REM sleep rebound and sleep patterns immediately returned to their pre-drug form. Its major clinical drawback was its short duration of action: its hypnotic effect lasting only 2 to 3 hr. We suggest that GHB may serve as the prototype for a new class of hypnotic compounds derived from natural sources and capable of activating the neurological mechanisms of normal human sleep.