The actions of cannabidiol (CBD), one of the cannabis constituents, were assessed on the sleep-wakefulness cycle of male Wistar rats. During acute experiments, single doses of 20 mg/kg CBD decreased slow-wave sleep (SWS) latency. After 40 mg/kg SWS time was significantly increased while wakefulness was decreased. REM sleep was not significantly modified. Following the once-daily injections of 40 mg/kg CBD for a period of 15 days, tolerance developed to all the above-mentioned effects.