Focal stimulation, stimulation of the fimbria, and stimulation of the medial septal area result in an inhibitory postsynaptic potential (IPSP) in lateral septal neurons. Increased stimulus intensity results in the appearance of a late hyperpolarizing potential (LHP). Treatment of the slice with bicuculline methiodide or picrotoxin results in blockade of the IPSP. When present, LHPs are enhanced in the presence of bicuculline or picrotoxin. Spontaneous and evoked IPSPs reverse near -70 mV, and LHPs reverse near -90 mV. Iontophoretic application of gamma-amino-butyric acid (GABA) results in hyperpolarizing, depolarizing, or biphasic potentials. Treatment with bicuculline or picrotoxin results in depression of biphasic GABA responses that appears selective for the depolarizing portion of the potential. At high concentrations of bicuculline, a portion of the hyperpolarizing GABA potential persists. The reversal potential of the depolarizing GABA potential is near -30 mV, and the reversal potential of monophasic hyperpolarizing GABA potential is near -70 mV. The bicuculline-resistant hyperpolarizing GABA response has a reversal potential near -90 mV. GABA activates three separate conductances on septal neurons, which are similar to those reported on hippocampal neurons. The resistance of the hyperpolarizing GABA potential to bicuculline appears to be due to the presence of a GABA-activated potassium conductance, which is similar to that activated by baclofen.