(1) Spontaneous firing of lateralis intermedius-lateralis posterior (LI-LP) thalamic output cells, as well as excitatory-inhibitory sequences elicited in LI-LP neurons by anterior suprasylvian cortical and center median (CM) thalamic stimulation were studied in encéphale isolé preparations during control periods of EEG synchronization and during EEG activation induced by stimulating the mesencephalic reticular formation (RF). (2) Mean rate of spontaneous firing of identified thalamocortical neurons increased during spontaneous or RF-induced EEG activation and decreased during EEG synchronization, occuring spontaneously or elicited by hypocapnia. Compared with the sustained firing during EEG activation, discharge patterns of output cells during EEG synchronization were characterized by very short (less than 5 msec) and very long (greater than 1 sec) interspike intervals, and by a class of intervals reflecting rhythmic (2.5-4/sec) periods of neuronal silence between high-frequency spike bursts. (d) The following effects were exerted by a short conditioning mesencephalic RF stimulation on cortically evoked events in single LI or LP thalamic neurons: (a) increased probability of the early, antidromically or orthodromically elicited discharges; (b) decreased probability of late (15-25 msec) discharges; (c) shortened latency of the subsequent inhibitory period; (d) reduction in the duration of the first inhibitory phase, and blocking of subsequent, cyclic inhibitory pauses as well as disorganization of postinhibitory clustered excitation.