1. Fifty-four neurones of the caudal part of the nucleus reticularis thalami (nuc. ret.) were recorded during different phases of sleep and wakefulness in unanaesthetized freely moving cats.2. During wakefulness the activity of the neurones was characterized by a continuous, well-spaced discharge. The mean firing rate was 35.58 +/- 15.06 spikes/sec (average +/- S.D.).3. During sleep with synchronized e.e.g. (S-sleep) the neurones fired in high frequency bursts with long pauses in between. Each burst was formed of 10-15 spikes. Often the bursts were followed by prolonged discharges formed of spikes well spaced one from the other. Bursts followed by prolonged activity were more commonly observed at the beginning of S-sleep and during the S-sleep periods preceding sleep with desynchronized e.e.g., whereas bursts immediately followed by silence were more frequent in the S-sleep periods with e.e.g. delta waves. The long periods of silence between the bursts usually lasted over 200 msec and values greater than 1 sec were frequently found. The mean firing rate of neurones during S-sleep was 19.22 +/- 10.50 spikes/sec.4. During sleep with desynchronized e.e.g. (D-sleep) the activity of the neurones was, as during wakefulness, characterized by a continuous, well spaced, unclustered discharge. The mean firing rate was 40.00 +/- 18.74 spikes/sec. During the rapid eye movements of this phase most units increased the frequency of their discharge, which, nevertheless, maintained the unclustered feature proper to the desynchronized phase of sleep.5. Interspike interval distribution was similar during wakefulness and sleep with desynchronized e.e.g., whereas that for sleep with synchronized e.e.g. was markedly different from those for both the other stages.6. The implications of the striking similarity between the activity of reticularis neurones during wakefulness and sleep with desynchronized e.e.g. are discussed.