The rhythms of spontaneously occurring seizures (spike-wave discharges, SWD) and motor activity, as well as the relationship between SWD and sleep-wake states were investigated in the WAG/Rij rat model of absence epilepsy. In order to establish whether SWD are controlled by external (Zeitgebers) or by endogenous factors such as circadian influences or the state of vigilance, the study was performed in entrained and constant dim light conditions. EEG and motor activity were recorded in the 12:12 light-dark cycle and in constant dim light conditions. Circadian rhythmicity was found both for motor activity and the occurrence of SWD in conditions of entrainment. In constant dim light conditions also circadian rhythms emerged, however, the change in circadian parameters was opposite for the rhythm of SWD and motor activity. SWD were preceded mostly by passive wakefulness and by slow-wave sleep in both experimental conditions. It can be concluded that the rhythm of SWD seems to be generated and controlled by an endogenous mechanism distinct from that which controls the rhythm of motor activity. The relationship between SWD and sleep-wake states preceding their occurrences appeared to be unchanged, suggesting that the mechanism of generation of SWD is independent of the circadian timing system.