Progestin receptors have been detected in the brain and pituitary of gonadectomized hamsters using the synthetic ligand (3H)R5020. The number of receptors found in males and females could be dramatically increased by priming the animals with estradiol benzoate 48 h before the binding was measured. Total progestin-binding activity in the cortex, hypothalamus-preoptic area and pituitary of both sexes was the same when high doses of estrogen (30 µg per animal) were used for induction, but the inducibility of the male receptors was found to decline with time after gonadectomy. These findings are discussed in relation to the effects of progesterone on aggressive and sexual behaviors in gonadectomized hamsters