Intracellular recordings of 31 lateral geniculate nucleus relay neurons were performed in darkness in behaving cats in order to analyse electrical postsynaptic events which appeared during slow-wave sleep. A specific pattern characterized slow-wave sleep: a rapid depolarizing potential arising from baseline initiated a slow depolarization lasting for 40-60 ms which in turn most often elicited delayed fast spikes. This pattern recurred at a frequency of 6-12/s. The slow depolarizations were voltage dependent, usually not separated by any obvious phasic hyperpolarization and showed refractoriness. Other rapid depolarizing potentials occurring during the time course or at the end of a slow depolarization could have generated spike(s) but were followed by a rapid decay. Slow depolarizations were not observed during arousal or paradoxical sleep when the neurons tonically depolarized and displayed either rapid depolarizing potentials with a fast decay or repetitive firing and long high frequency bursts. In five of the studied neurons, decreases in frequency of the spontaneous rapid depolarizing potentials occurred during slow-wave sleep for 3-30 s oscillatory periods without any change in the behavioural state. During these periods all of the few remaining rapid depolarizing potentials arose from a flat baseline, had a higher amplitude and initiated a slow depolarization which always elicited a spike or burst of spikes after a brief delay. The slow-wave sleep rhythm decreased to 1-5/s. Simultaneously the baseline membrane potential hyperpolarized by a few millivolts and reached a level for reversal of inhibitory postsynaptic potentials. Imposed hyperpolarization of the membrane during wakefulness did not reveal any slow depolarization. But strong synaptic excitatory inputs and direct excitation (a break of the current pulse) from a hyperpolarized membrane did evoke the slow depolarization and eventually the fast spike(s) in both control and oscillatory neurons. A rhythm similar to that of slow-wave sleep was elicited during wakefulness by optic tract stimulation and was enhanced by membrane hyperpolarization. But under these conditions the rhythm was initiated by a phasic hyperpolarization and was composed of an alternating hyperpolarization-depolarization. Spontaneously and synaptically evoked rapid depolarizing potentials arising from baseline had a similar rising slope. The spontaneous ones initiated a slow depolarization leading to fast spike(s) during slow-wave sleep and could directly generate fast spike(s) during wakefulness.(ABSTRACT TRUNCATED AT 400 WORDS)