The correlation between discharges of a penicillin focus and unit activity was studied in the sensorimotor cortex of 30 adult male curarized rats under urethane anaesthesia. Among 43 units, three types of activity could be differentiated: (1) units without any relationship to focal discharges (7% of neurones); (2) units with a biphasic reaction, a burst of spikes during the main negative deflection of the focal discharge followed by a period of inhibition (72%); (3) units inhibited during the whole duration of the focal discharge (21%). The effects of diphenylhydantoin (60 mg/kg i.p.) were studied on 35 units--20 of them were also recorded before administration of diphenylhydantoin and thus included in the above group. Diphenylhydantoin led to decreased representation in groups (1) and (2), whereas the percentage of units inhibited during the focal discharge was increased from 21 to 31%. A decrease in number of spikes in the excitatory burst and/or the prolongation of the inhibitory phase was a consequence of diphenylhydantoin administration in 18 out of 20 neurones with long-lasting recording. A direct action of diphenylhydantoin on the primary epileptogenic focus was thus demonstrated.