Diazepam (5 x 10(-8) - 10(-6) M) was found to augment recurrent inhibition of pyramidal neuron firing in a dose-dependent manner in rat hippocampal slices. To determine possible loci of this effect, diazepam was locally administered by pressure ejection from a micropipette, while recording action potentials from single inhibitory ("basket") interneurons. Diazepam induced reversible and reliable increases in interneuron firing in response to stimulation of Schaffer collateral and commissural afferents. Taken together with previous electrophysiological reports, these data suggest that benzodiazepines may augment central inhibition by increasing either the excitability of inhibitory interneurons, or by increasing the strength of excitatory afferents to these cells.