p-Bromomethamphetamine (V-111) was given as a single dose to one group of rats and for 28 days to another group and its influence was studied on the sleep-walking cycle. In a third group of rats the midbrain raphe nuclei was lesioned before administration of the drug. A single dose of 15 mg/kg s.c. V-111 produced total insomnia, the SWS returned 15-16 h, REM sleep 21-22 h after the injection. When the drug was given repeatedly the periods of insomnia were gradually shortened. At the end of third week the SWS returned in daytime after 3 h, at night after 6 h, REM sleep after 6 h and 8 h respectively. The destruction of the midbrain raphe nuclei failed to influence either the spontaneous sleep patterns or the insomnia elicited by V-111. In physiological conditions the integrity of 5-HT system is necessary for the regulation of sleep-waking cycle. After reduction of brain 5-HT, insomnia developed but the sleep-waking cycle could be restored while the brain 5-HT was decreased. The restored sleep mechanisms seem to be 5-HT dependent also.