The timing of amygdala kindled and penicillin seizures was studied throughout the sleep-wake cycle in eight cats following near total sleep deprivation and a control procedure that did not affect sleep time. Sleep loss was induced by 24-h exposure to a modified "flower pot" procedure employing a small pedestal. The control procedure consisted of 24-h exposure to a larger pedestal. Sleep loss increased susceptibility to generalized kindled and penicillin seizures during all waking and sleep states but did not alter temporal patterns of seizure susceptibility. Both before and after sleep loss, kindled cats showed maximal seizure susceptibility, indexed by lowest seizure thresholds, during slow wave sleep (SWS) and transitions from SWS to REM sleep (REMS). The spike-wave discharges and motor seizures of systemic penicillin epilepsy were always most frequent during SWS and during drowsiness after awakening. Both models were invariably most resistant to seizures during stable REMS. To explain sleep- and sleep loss-activated seizures, we summarized previous work suggesting that sleep abnormalities dictate the timing of seizures and are exacerbated by sleep loss. Abnormal behavioral arousals and pathological somatomotor system excitability occur in both models and are particularly pronounced during seizure-prone intervals. Sleep loss may magnify somatomotor system hyperexcitability patterns in all states, thus allowing abnormal motor arousals and seizures to intrude during seizure-prone and seizure-resistant sleep and waking states.