When the MtTF4 pituitary tumor was grown in male rats not treated by estradiol, the cytosol prepared from it contained very few binding sites for the synthetic [3H]ORG progestin. Following estradiol treatment there was a reversible increase of these binding sites. The ligand specificity and the sedimentation constant were shown to be similar to those of progestin receptors contained in other tissues. Daily treatment for 8 days with 10 micrograms 17 alpha-estradiol, 50 micrograms dihydrotestosterone or 50 micrograms dexamethasone did not induce progestin binding. CONCLUSIONS (1) the virtual absence of progestin receptors in a tumor does not allow an absence of response to endocrine manipulation to be predicted; (2) the progestin receptors are induced by estradiol in the MtTF4 tumor despite the fact that the growth of this tumor is inhibited by estradiol; and (3) this work, taken together with previous reports, suggests that the estrogen receptors known to be located in the MtTF4 tumor have some functional properties in common with those tissues or cells whose growth is either stimulated by or insensitive to estradiol.