Auditory and somesthesic forms of stimulation have substantially increased rapid eye movement (REM) sleep in cats. We investigated whether auditory stimulation, applied during REM sleep or outside REM sleep, would have similar effects in normal volunteers. We also administered auditory stimulation to depressed patients during REM sleep. Subjects were studied during 1 acclimatization night, 2 baseline nights, 4 consecutive nights with auditory stimulation, and 1 followup night without auditory stimulation. Normal volunteers were randomly divided into Group R, which received auditory stimulation during each REM sleep episode, and Group NR, which received auditory stimulation at the end of each REM sleep episode. Depressed patients (Group D) received auditory stimulation during each REM sleep period. Only Group R showed increased REM sleep time during the nights of auditory stimulation and throughout the followup night. This group also increased their sleep efficiency. Group NR showed reduced sleep efficiency due to an increase in both the duration and frequency of awakenings. Group D did not show increased REM sleep time, but did show shortened REM sleep episodes, increased REM sleep frequency, and increased duration of awakenings. Group D did not show clinical changes.