Unit discharges were extracellularly recorded from antidromically identified thalamocortical neurones of ventralis lateralis (v.l.) and centralis lateralis (c.l.) nuclei as well as from reticularis thalami (re.) neurones during wakefulness and electroencephalogram-synchronized sleep of the behaving cat. Various parameters of sleep-related discharge bursts were analysed. Statistical analyses revealed striking similarities between motor relay (v.l.) and intralaminar (c.l.) neurones. More than 60% of bursts consist of three to five spikes at 250-400 Hz. The defining feature of bursts in all cortically projecting neurones is a progressive increase in the duration of successive interspike intervals. As in thalamocortical cells, all re. neurones change their tonic discharges in waking to bursting firing in sleep, regardless of the increased or decreased firing rates from wake to sleep in individual neurones. The bursts of re. neurones are essentially different from those of thalamocortical cells. In re. neurones, burst structure consists of an initial progressive decrease in duration of interspike intervals, followed by an increase in duration of successive intervals, eventually leading to a long-lasting tonic spike train at about 100 Hz. In contrast with bursts of thalamocortical neurones, only 6% of re. bursts are shorter than 50 ms; the total duration of the burst extends between 50 ms and 1.5 s. Population periburst histograms show the beginning of a decline in firing probability about 1.5 s prior to burst onset and an increased firing probability persisting for 300-350 ms after burst onset. The different electrophysiological properties underlying the burst structure of cat's thalamocortical and re. neurones are discussed, with emphasis on dissimilar aspects of re. bursts in unanaesthetized and barbituratized preparations. Various factors that may account for the transition from tonic mode in waking to bursting mode in sleep are envisaged.