Intracellular recordings were performed in the CA1 region of the rat hippocampus following an ipsilateral intraventricular injection of kainic acid. Seven days postlesion, graded bursts of up to four action potentials could be evoked by stimulation of the stratum radiatum. The evoked EPSPs underlying these bursts showed a prolonged 10–90% rise time and half-width compared to control EPSPs, an absence of a significant inhibitory phase, and an increase in magnitude and duration at depolarized resting levels. The evoked EPSPs also exhibited a significant decrease in amplitude and time course in response to D-APV (D-2-amino-5-phosphonovaleric acid; 1–20 microM), though this effect was variable from cell to cell. The prolonged time course, voltage sensitivity, and response to a selective NMDA antagonist confirmed that the major component of the EPSP in neurons from lesioned slices was mediated by NMDA receptors. The partial denervation of the CA1 area induced by the kainic acid led to both an enhanced NMDA-mediated excitatory phase and a decrease in postsynaptic inhibition, resulting in the pronounced hyperexcitability noted in the lesioned slices.