Skin potential levels and EEG changes were recorded in eight psychiatric patients during three nights of sleep. In a balanced design each patient took amylobarbitone sodium 200 mg, chlordiazepoxide 30 mg and placebo in turn. Skin potential did not distinguish between wakefulness and sleep as measured by the EEG nor did it clearly identify individual sleep stages. However, significant differences in skin potential were found between Awake and Stage I, between Awake and REM sleep and between REM sleep and Stage III/IV. The level was lowest during REM sleep but approached that of wakefulness during slow-wave sleep. These findings are discussed in terms of changes in arousal threshold during sleep. A cautious comment is made on the possible effects of psychiatric diagnoses and drugs.