The regulation of <sup>3</sup>H-muscimol binding to high-affinity GABAa receptors by estradiol (E) and by progesterone (P) was studied within discrete brain regions using in vitro quantitative autoradiography. Treatment of ovariectomized and adrenalectomized female rats with E resulted in a decrease of muscimol binding only in specific estrogen-sensitive brain regions like the ventromedial nuclei (VMN) of the hypothalamus, the arcuate nucleus (ARC), the medial amygdala and the midbrain central grey (MCG). When administered alone, P had no effect. However, in estrogen-primed females, P increased muscimol binding in both VMN and MCG to levels seen in control animals. Thus, E and P exert opposite effects on the the GABAa receptor within these two nuclei. As both hormones facilitate female reproductive behavior as well as the release of luteinizing hormone, present results suggest that E and P affect muscimol binding by different mechanisms.