WE combined histofluorescence within sit uhybridization to identify GABAergic neurons in the arcuate nucleus (ARC) following electrophysiological recordings, using GAD65as a marker. Intracellular recordings were made in hypothalamic slices prepared from ovariectomized guinea pigs. Over 90% of ARC neurons tested with the GABABreceptor agonist baclofen responded with a membrane hyperpolarization or an outward current. The hyperpolarization was dose-dependent, and the GABABreceptor antagonist CGP 35,348 produced a rightward shift in the agonist dose-response curve. Agonist potency was lower, and the efficacy greater, in GAD-positive neurons. The use of this novel technique for identifying GABAergic neurons thus reveals differences in the pharmacodynamics of GABABreceptor activation GABAergic and non-GABAergic ARC neurons