The following properties of the cytoplasmic progestin receptor were studied in the hypothalamus, cortex, pituitary and uterus of the estrogen-primed castrated adult female rat using the highly potent progestin R 5020 (promegestone). (a) Sedimentation pattern. In sucrose density gradients, the R 5020-progestin receptor complex sedimented with a coefficient of about 6 to 7S. (b) Binding parameters. R 5020 bound to the progestin receptor with an intrinsic dissociation constant of about 10(-9) M as measured by a Dextran-coated charcoal (DCC) technique. The number of binding sites, however, differed widely. (c) Specificity. Only progestins competed for [3H]R 5020 binding. (d) Estrogen-dependency. In both immature and castrated adult rats, estrogen administration increased the number of R 5020-specific binding sites, assayed in vitro by a DCC technique, in the uterus, pituitary and hypothalamus, but not in the amygdala, hippocampus nor in the cortex. The increase was maximum between 40 and 48 h after priming with the potent estrogen, moxestrol, and could not be induced by androgens nor by progestins.