Lindane, 10 mg/kg, IP in DMSO, was administered to rats implanted with electrodes to stimulate the perforant path and record from the dentate gyrus of the hippocampal formation. Dentate responses were examined in acutely prepared, anesthetized rats and chronically prepared rats in unanesthetized and anesthetized states. The most significant and reliable effect of lindane administration was to increase the number of granule cells excited to discharge in response to perforant path stimulation. Further analysis indicated that lindane did not appear to produce this action by increasing transmitter release from the perforant path terminals. The primary site of action was the granule cell itself. Inhibition, mediated by recurrent collaterals and believed to be GABA-mediated, was not reduced and, in some circumstances, was increased after lindane administration. These data suggest that the primary action of lindane in intact subjects is to increase neuronal excitability. Enhanced transmitter release or reduction in GABA-mediated inhibition could not be demonstrated to contribute significantly to this effect.