Superfusion of ethanol (10-350 mM) sometimes caused weak hyperpolarization, but more often elicited weak depolarization or biphasic depolarizing, hyperpolarizing responses in CA1 and CA3 pyramidal neurons of the hippocampal slice. The occasional polarizations were sometimes accompanied by, but not always correlated with, small increases or decreases in input resistance. However, many cells in both areas showed no detectable change in membrane potential (36% of cells) or input resistance (57% of cells), even at very high ethanol concentrations (86-200 mM). Spontaneous spiking, when present, was occasionally accelerated or decelerated, although in CA3 a biphasic speeding-slowing sequence was often seen. The afterhyperpolarizations following bursts of action potentials evoked by current (CA1) or occurring spontaneously (CA3) were most often either slightly reduced in amplitude (CA3) or not affected (CA1) by ethanol superfusion. In contrast, synaptic potentials evoked by stimulation of the hilar mossy fiber pathway (for CA3) or the stratum radiatum (for CA1) were more sensitive to ethanol: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were most often reduced in amplitude in both CA1 and CA2, even at low ethanol concentrations (10-50 mM). The action on IPSPs may be exerted presynaptically, because responses to locally applied GABA were little affected. These results suggest that hippocampal evoked synaptic activity may be more sensitive than postsynaptic membrane properties to physiologically relevant ethanol concentrations.