The effects of estradiol benzoate (EB) and of progesterone (P) treatment on high-affinity [3H]muscimol binding in the dorsal hippocampal formation were examined in ovariectomized and adrenalectomized female rats by in vitro autoradiography. EB injected subcutaneously increased [3H]muscimol binding in specific subregions of the Ammon's horn (CA1 and stratum radiatum of CA4) and of the dentate gyrus (dorsal molecular layer). In these particular regions, estrogen receptors have been shown to be present. P did not significantly affect [3H]muscimol binding in any region of the hippocampus when administered alone or in combination with EB. Results suggest that estrogens may regulate the activity of specific hippocampal neurons by modulating their sensitivity to GABA.