Kainic acid lesions of hippocampal subfields CA3-CA4 produced dramatic synchronous afterdischarge activity in subfield CA1 when studied 2-4 weeks post-lesion in the in vitro slice preparation. This epileptiform discharge was correlated with a loss of intrinsic firing-induced afterhyperpolarizations and synaptic IPSPs. Two to 4 months post-lesion, intrinsic afterpotentials and synaptic inhibition appeared normal in most cells studied.