Effects of different photoperiods on the appearance of EEG slow waves were examined in freely-moving chickens by a radio telemetry system. The experiments were performed under 14L10D, 18L6D, 21L3D and 24L. It was clear that the EEG components were strictly synchronized to light-dark cycles. Continuous illumination exerted a dampening effect on the appearance of the slow wave diurnal rhythms. Chickens exposed to light-dark cycles of 14L10D and 18L6D maintained a constant daily level of slow wave activity. These levels are regarded as a normal amount of slow wave activity in male chickens. The daily amount of slow wave activity under 21L3D and 24L is probably regulated in a way different from that under 14L10D and 18L6D. The illumination seems to exert a strong effect on the mechanism controlling the appearance of the EEG in chickens when compared to mammals.