Phenytoin (10-100 microM) was studied on excitatory synaptic transmission and post-tetanic potentiation (PTP) in the in vitro rat hippocampus. Synaptic potentials were studied using extracellular, intracellular and single-electrode voltage clamp techniques. Field excitatory postsynaptic potentials were recorded from the apical dendrites of CA1 pyramidal cells after Schaffer collateral stimulation. Intracellularly recorded excitatory postsynaptic potentials and excitatory postsynaptic currents were recorded in CA3 pyramidal cells after mossy fiber stimulation and in the presence of 10 microM picrotoxinin. In the CA1 region, phenytoin elicited a reversible depression of field excitatory postsynaptic potentials as well as reduced the time constant of decay of PTP from 79 sec to 47 sec with no change in the magnitude of potentiation. Higher concentrations of phenytoin (100 microM) had a general depressant effect on both the amplitude and time course of PTP. In CA3 cells, phenytoin (10 microM) reduced the mossy fiber synaptic conductance but did not change its reversal potential. Phenytoin (10 microM) also reduced the time constant of decay of PTP of the mossy fiber to CA3 synapse, while having no effect on the magnitude of potentiation. These results show that therapeutically relevant concentrations of phenytoin depress both low-frequency synaptic transmission and the time course of short-term potentiation. Both actions may be involved in the anticonvulsant properties of phenytoin.