In epilept WAG/Rij rats, multiple unit activity coinciding with the occurrence of spike-wave discharges was recorded under neurolept anesthesia. Recordings were made in the frontal cortex and in various nuclei of the thalamus, in specific nuclei such as the ventroposterolateral, the ventroposteromedial and the ventrolateral nuclei, as well as in non-specific nuclei such as the mediodorsal nucleus, the reticular thalamic nucleus, the interanteromedial nucleus and the intralaminar nuclei (the central medial nucleus, the centrolateral nucleus and the paracentral nucleus). Rhythmic unit firing concurrent with the spike component of the cortical spike-wave discharge was observed in deep layers of the cortex and in the following thalamic nuclei: in specific nuclei, the mediodorsal and the reticular thalamic nucleus. The activity in the specific nuclei and the mediodorsal nucleus shortly preceded the peak of the spike component. The burst in the reticular thalamic nucleus occurred later than in the specific nuclei. A wave-concurrent firing pattern was observed in the centrolateral nucleus and the paracentral nucleus. Cells in the central medial nucleus and interanteromedial nucleus did not fire in a phase-locked manner. Neurons in the latter nucleus, however, were generally tonically activated during the occurrence of spike-wave discharges. It is suggested that those thalamic nucleic thought to be involved in the production of cortical spindles, and that also fire concurrently with the spike component of the spike-wave discharges, are mediated in the genesis of the latter activity.(ABSTRACT TRUNCATED AT 250 WORDS)