A slow oscillation (< 1 Hz) has recently been described in intracellular recordings from the neocortex and thalamus (Steriade et al., 1993c-e). The aim of the present study was to determine the phase relations between cortical and thalamic neuronal activities during the slow EEG oscillation. Intracellular recordings were performed in anesthetized cats from neurons in motor and somatosensory cortical areas, the rostrolateral sector of the reticular (RE) thalamic nucleus, and thalamocortical (TC) cells from ventrolateral (VL) nucleus. The EEG was used as time reference for alignment of activities in different, simultaneously recorded neurons, including dual impalements of cortical cells as well as cortical and TC cells. The spontaneous EEG oscillation was characterized by slowly recurring (0.3–0.9 Hz) sequences of surface- positive (depth-negative) sharp deflections, often followed by oscillatory activity within the frequency range of sleep spindles (7–14 Hz) or at faster frequencies. Cortical and RE cells were similarly hyperpolarized during the depth-positive EEG waves and were depolarized during the depth-negative EEG deflections. In many instances, the cell depolarization was associated with oscillations at the spindle frequency or with tonic firing at rates related to the level of depolarization. TC neurons were hyperpolarized during the depth- positive EEG waves and displayed a series of IPSPs, at the spindle frequencies, during the depth-negative EEG waves. Depending on the membrane potential (Vm), TC cells could fire spike bursts at the onset of the EEG depth-negativity, or their firing could be delayed by subsequent IPSPs. The sequence of spontaneous EEG and cellular events described above also characterized the responses to cortical and thalamic stimulation. Simultaneous intracellular recordings of pairs of cortical cells or cortical and TC cells showed that spontaneous transitions from less synchronized to more synchronized EEG states were marked by a simultaneous hyperpolarization, coincident with an overt depth-positive EEG wave. We conclude that during low-frequency oscillatory states, characteristic of slow-wave sleep, neocortical and thalamic neurons display phase relations that are restricted to narrow time windows, and that synchronization results from a generalized inhibitory phenomenon. Moreover, EEG synchronization is reflected as active inhibition in TC neurons. That this pattern is also present in states of hypersynchronization, such as seizure activity, is shown in the following paper (Steriade and Contreras, 1994).