To investigate GABAergic inhibitory inputs to neurons of the subfornical organ (SFO), intracellular recordings were made in rat brain slice preparations. Inhibitory postsynaptic potentials, which occurred spontaneously or were evoked by focal electric stimulation, had reversal potentials of approximately −60 mV, and were almost totally abolished by the GABA A antagonists bicuculline at 3–100 μM or picrotoxin at 50 μM. Following the application of bicuculline or picrotoxin, the resting membrane potentials were decreased by 4–8 mV. GABA at 10–100 μM and the GABA A agonist muscimol at 1–100 μM decreased the membrane resistance and the firing rate in all neurons tested. The reversal potential of the response to muscimol was similar to that for inhibitory postsynaptic potentials. The actions of muscimol persisted in the presence of 1 μM tetrodotoxin, implying that muscimol must act directly on the recorded neurons. These results suggest that there is a tonic inhibitory GABAergic input to SFO neurons which are mainly mediated through GABA A receptors.