Progesterone-binding components were detected in hypothalamic and anterior hypophysial cytosols from estrogen-primed immature and mature female rats. These components were labeled in vitro with a tritiated synthetic progestin, R5020 [17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione] which binds specifically to progesterone receptors. The radioactive complex sediments in the 7S region in sucrose density gradients containing 10% glycerol. Little 7S binding was detected in similar preparations from the cerebral cortex, amygdala or reticular formation. In contrast with the marked effects by the estrogen-priming, very low or no 7S binding was found in the hypothalamus and hypophysis from intact nonprimed immature female rats. The dissociation constants (Kd) and the number of binding sites (NBS) of the hypothalamic components in immature rats were 3.3 .times. 10-9 M and 1.0 .times. 10-14 mol/mg cytosol protein, respectively. These values for the adult rat anterior hypophysis were 3.8 .times. 10-9 M and 3.1 .times. 10-14 mol/mg cytosol protein, respectively. The 7S binding components in the hypothalamus and hypophysis were specific for progestational compounds. 5.alpha.-Dihydroprogesterone greatly lowered the 7S binding in both the tissues, but its 5.beta.-isomer was without effect. Among steroid competitors tested, only estradiol competed weakly. Incubation with pronase abolished the 7S R5020 binding. Heat experiments revealed the thermolabile nature of the components. In addition to the 7S binding component, a 4S complex of bound R5020 was evident in the hypothalamus and cerebral cortex but not in the hypophysis. This binding seemed to be mostly nonspecific in nature. The cytosols from the hypothalamus and anterior hypophysis of estrogen-primed female rats contain specific progesterone receptors.