The effects of non-convulsive doses of i.v. penicillin (1.100.000/1.300.000 I.U./Kg) on the ECoG of rabbits was studied. A tendency to an increase in spindling activity together with the presence of characteristic penicillin spikes, polyspikes and spike and wave complexes often preceding or intermixed to the spindling activity, was the most prominent finding. These data indicate that spindles and penicillin induced spikes show a straight correlation and seem consistent with the hypothesis that spindles have a facilitant effect on epileptic discharges.