Gastric acid secretion during natural sleep was studied in 4 healthy female volunteers for 11 nights. Acid output was measured by means of intragastric titration and a telemetering capsule, and sleep was monitored continuously by recording EEG and eye movements. Compared to the waking state, sleep was found to be associated with significantly lower levels of acid secretion. Although there were no significant differences between acid secretion during sleep stages 1 to 4 and rapid eye movement (REM), acid secretion decreased with deeper stages of sleep. During all REM phases only small amounts of acid were produced. Arousal or periods of waking in the course of the night, as well as waking in the morning, were associated with an increase in acid output.