The effects of ethanol on synaptic transmission in hippocampus were studied by changes in the response of CA3 pyramidal cells to stimulation of two afferent pathways, the dentate (mossy fiber) pathway, and the commissural pathway. Both sources produce an excitatory response as measured either by single unit spiking or by population spike followed by a period of post-stimulus inhibition (PSI). After the injection of 3 g/kg ethanol (i.p.), both the excitatory responses and the duration of PSI are significantly increased. Because these changes occur in both afferent pathways, they are not pathway specific, but may be the result of the local microcircuitry in area CA3. Although the change in excitatory and inhibitory responses can occur simultaneously, detailed statistical analyses show that neither the magnitude nor onset times are correlated. Thus, the two responses are functionally separable. In addition, the increase in the duration of PSI is related to the rate of rise of blood ethanol level and shows short-term tolerance. The maximum change in the PSI occurs after blood ethanol levels plateau suggesting a secondary process is necessary before neurophysiological effects are apparent.