Protein kinase C (PKC) is present abundantly in the mammalian central nervous system, and is involved in a variety of neuronal functions. Phorbol esters mimic the role of diacylglycerol, the physiological activator of PKC. We examined effects of phorbol 12,13-diacetate (PDAc) on excitatory synaptic transmission in neurons in the dentate granule cell layer of rat hippocampal slices using the whole-cell patch clamp technique. Excitatory postsynaptic currents (EPSCs) evoked by stimulation of the perforant path (pp) consisted of AMPA and NMDA receptor-mediated components. The application of PDAc potentiated both components of the EPSC, but the effect was more pronounced on the NMDA component. The potentiating effect of PDAc on the NMDA component was dependent on the membrane potential, being most prominent at - 31 and -51 mV. Omega-agatoxin-IVA, a P-type Ca2+ channel blocker, suppressed both AMPA and NMDA components to a similar extent by reducing transmitter release. However, when the PDAc-potentiated AMPA component was reduced to the control level by applying omega-agatoxin-IVA, a substantial potentiation on the NMDA component remained. These results suggest that the potentiation of the NMDA component of the EPSC by PDAc is caused partly by a postsynaptic mechanism in the dentate neurons.