Enkephalin analogues were superfused onto hippocampal slices during intracellular recording of dentate granule cells. The enkephalins elicited either weak depolarizations, weak hyperpolarizations, or no effect on membrane potential, in about equal numbers of cells tested. Similarly, input resistance either decreased, increased or did not change, and was not well correlated with the potential changes. However, at all concentrations tested (2-10 microM) the enkephalins reduced evoked depolarizing synaptic potentials by up to 70%. We speculate that a major function of endogenous enkephalin-containing fibers projecting to the dentate is to dampen afferent synaptic transmission.