Estrogen target neurons are numerous in the medial preoptic/anterior hypothalamic area (MPO/AH) of the female rat brain, and they are thought to play a crucial role in reproductive functions. This brain region is also known to contain high concentrations of the inhibitory transmitter gamma-aminobutyric acid (GABA) and of its synthesizing enzyme glutamate decarboxylase (GAD). Since it is known that GABA is involved in the regulation of gonadotropin release from the pituitary gland it has been proposed that estrogen feedback may be mediated by this transmitter. Here we show, by a combined method of estrogen autoradiography and GAD immunocytochemistry, that estrogen-receptive neurons of GABAergic nature exist in the MPO/AH.