The effect of intraperitoneally injected delta-sleep-inducing peptide (DSIP) on sleep-wakefulness in cats was studied using EEG, EMG and EOG recording for 10 h following 30 nmol/kg DSIP or control saline i.p. injections. DSIP reduced the amount of sleep, specifically light slow-wave sleep and REM sleep, and REM sleep latency was increased. The results suggest that in cats with redundancy sleep DSIP increases wakefulness at the cost of light slow-wave sleep, and in addition it has a specific REM-reducing effect.