Periods of disfacilitation associated with EEG depth-positive waves and long-lasting hyperpolarization of cortical and thalamic neurons are essential features of slow wave sleep. Electrical stimulation of prethalamic pathways in cats during EEG depth-positivity fails to elicit cortical responses (Timofeev et al., 1996, J. Physiol). Recent human studies showed a modulation of somatosensory evoked potential by the slow sleep oscillation, but the responses were not abolished at any given phase of the slow sleep oscillation (Massimini et al., 2001, SFN abstr.). In the present study conducted on anesthetized cats we investigated the mechanisms of these differences. We found that electrical stimulation of contralateral forelimb elicits variable cellular and field potential responses in somatosensory cortex in all phases of the slow oscillation. The responses to electrical stimulation of medial lemniscus were significantly decreased or abolished during EEG depth-positive waves. Intracellular recordings from VPL n...