Pathological somatomotor system excitability and generalized seizures occur throughout the sleep-awake cycle but peak at different times in the amygdala kindling and systemic penicillin epilepsy models. Sleep loss increases seizure activity in both models during all waking and sleep states but does not alter the timing of seizure susceptibility in the sleep-wake cycle. Although the mechanism for sleep-deprivation seizures is unknown, we propose that sleep loss magnifies somatomotor system hyperexcitability patterns in all states, thereby increasing seizure vulnerability at all times but preferentially during seizure-prone intervals. To evaluate this hypothesis, the timing of ventral lateral thalamic and motor cortex excitability, indexed by amplitudes of primary evoked responses, was studied throughout the sleep-wake cycle in eight cats before and after nearly total sleep deprivation. Sleep loss was induced by 24-h exposure to a modified "flower pot" procedure; the control procedure consisted of 24-h exposure to a larger pedestal which did not affect sleep time. Findings confirmed the hypothesis, as follows: (i) Sleep deprivation increased ventral lateral thalamic and motor cortical excitability nonspecifically. (ii) Motor cortex hyperexcitability correlated best with penicillin seizure activity; both were elevated in slow-wave sleep and drowsiness after awakening from slow-wave sleep before sleep loss and were further increased by sleep loss. (iii) Ventral lateral thalamic hyperexcitability patterns correlated best with the timing of kindled seizure susceptibility; both peaked during transitions from slow-wave to REM sleep before and after sleep loss but were maximal after sleep loss. (iv) Sleep loss increased thalamocortical excitability and seizure susceptibility during stable REM sleep in both models, but values were lower than in other states. These results suggest a chronic neuropathology at different levels of the neuraxis for dissimilar epilepsy models and upon which the sleep-waking state modulation of seizures is superimposed. Sleep deprivation may aggravate seizures in both models by nonspecific enhancement of thalamic and cortical excitability.