Intracellular recording in the in vitro hippocampal slice was utilized to examine the effects of nimodipine and nifedipine on CA1 pyramidal cell excitability. The excitatory postsynaptic potential (EPSP) elicited by a single stimulus in stratum radiatum was enhanced by nifedipine as evidenced by increases in EPSP amplitude, area and slope. Threshold for synaptically-evoked somatic action potentials was decreased following either nifedipine or nimodipine application, often resulting in spontaneous action potential activity. A secondary, late EPSP-like event appeared in the intracellular recordings during and following bath application of nimodipine, and was associated with burst-like activity in field potential recordings. In accordance with the hydrophobic nature of these compounds, extensive washout in normal Krebs' solution failed to reverse their effects, but nifedipine's actions were photolabile. These results indicate that dihydropyridines can enhance synaptic efficacy in the CA1 region of the hippocampus.