Repeated injections of delta sleep-inducing peptide (DSIP) were given to a 35-year-old male narcoleptic. Effects on wakefulness and sleep were evaluated by self-reports, performance tests, multiple sleep latency test and all-night polysomnography. DSIP reduced the frequency of sleep attacks and increased activity, alertness and performance during daytime. The sleep period was compressed by DSIP with enhancement of REM sleep. The results suggest that the effects are due to an accentuation of circadian and ultradian rhythms by DSIP.