Putative progestin receptors have been characterized in brain and pituitary tissue from untreated and estrogen-primed ovariectomized-adrenalectomized rats. The properties of these sites appear indistinguishable from those of cytoplasmic progestin receptors from the uterus: 1) sedimentation coefficient of 7S, which is reduced by half in the presence of 0.3 M KCl; 2) specificity of binding which strongly favors synthetic and natural progestins as opposed to glucocorticoids, androgens, and estrogens; 3) a dissociation constant for binding the synthetic progestin [3H]R5020 (17 alpha, 21-dimethyl-19-norpregna-4, 9-diene-3,20-dione) of 0.3 nM; and 4) similar rates of formation and dissociation of the [3H]R5020-receptor complexes. In these respects, the estrogen-inducible and noninducible receptors of the brain also appear to be indistinguishable from each other. Estrogen induction of progestin receptors is apparent in uterus (6-fold), pituitary (8-fold), mediobasal hypothalamus (4-fold), and preoptic area (4-fold), all estrogen receptor-containing areas. The corticomedial amygdala does not show an estrogen effect on progestin receptor levels even though it contains estrogen receptor sites. The midbrain of the rat doeogen-insensitive receptors in the brain is relatively low and of the same order of magnitude as in nonstimulated hypothalamus, preoptic area, and pituitary, yet variations are seen among the estrogen-insensitive structures, with lowest levels occurring in cerebellum (6-7 fmol/mg protein) and highest levels occuring in cerebral cortex (approximately 25 fmol/mg protein). These findings are discussed in relation to the actions of progesterone which do and do not require estrogen priming and in relation to intracranial progesterone implantation studies.