One mechanism whereby progesterone opposes the regulatory actions of estrogen in the brain may include the down-regulation of estrogen receptors. A previous study has shown a small decrease in estrogen binding in the combined preoptic area-hypothalamic area in response to progesterone treatment. To determine if this effect is region specific, gonadectomized/adrenalectomized (GDX/ADX) estrogen-treated female rats were administered a single injection of progesterone (5.0 mg, sc) or a silastic capsule containing crystalline progesterone. Control animals were treated identically but without progesterone exposure. Animals were killed 24 or 72 h after initiating progesterone treatment and estrogen binding was measured in cytosol and cell nuclear extracts of the anterior pituitary, mediobasal hypothalamus (MBH), and preoptic area. A significant effect of progesterone injection on nuclear estrogen binding was observed in the MBH at 24 h. This effect had subsided by 72 h. No effect was observed when progesterone was administered in a continuous fashion. To further examine the regional specificity of the progesterone effect and to determine if males were similarly affected, nuclear-bound estrogen receptors were measured in microdissected brain regions from male and female estrogen-treated GDX/ADX rats treated with 5.0 mg progesterone or vehicle 24 h before sacrifice. A significant decrease in estrogen binding was found in the ventromedial nucleus of the female but not the male. A significant effect of progesterone treatment was found in the periventricular preoptic area of the male.(ABSTRACT TRUNCATED AT 250 WORDS)