Intracellular recordings were obtained from CA1 neurons of hippocampal slices maintained in vitro. Epileptiform activity was induced by bath application of either penicillin (3.4 mM) or picrotoxin (5 × 10-4 M). The afterhyperpolarizations (AHPs) following evoked epileptiform events induced by penicillin were inverted by diffusion of chloride ions from KC1 electrodes but were insensitive to injection of EGTA. Picrotoxin produced epileptiform events qualitatively similar to those seen with penicillin, but the AHPs were reduced or abolished by injection of EGTA and were chloride insensitive. AHPs that were evoked directly by depolarizing-current pulses when penicillin or picrotoxin was present were sensitive to EGTA. These results indicate that hippocampal CA1 neurons are capable of generating two types of hyperpolarizing events and that penicillin is apparently ineffective in completely blocking the chloride-mediated event. © 1981.