The distribution of specific nuclear binding sites for androgens and estrogens in the male accessory sex organs of the mouse was assessed by autoradiography with3H dihydrotestosterone (3H DHT) and3H estradiol (3H E2). With3H DHT nuclear labeling differed among the epithelia of the organs. It was high in seminal vesicle and ampullary gland, moderate in ventral prostate, urethral gland, prostatic excretory ducts and the ampulla ductus deferentis, low in dorsal prostate and low or absent in coagulation gland. With3H E2, in contrast, epithelial nuclear labeling was high only in coagulation gland, moderate or low in seminal vesicle, low or absent in ventral and dorsal prostate and absent in ampullary gland and ampulla ductus deferentis. In the lamina propria of all organs nuclear labeling with3H DHT was generally moderate and existed only in some cells, with the highest number in the ampulla ductus deferentis. With3H E2, nuclear labeling in the lamina propria showed a high intensity in all organs, except in ventral and dorsal prostate which remained unlabeled. Many labeled cells were found in the deferent duct and its ampulla, while in the other organs only a few cells showed nuclear labeling with3H E2. In the smooth muscle sheath of all organs, some muscle cells were moderately labeled with3H DHT, but not with3H E2. The results indicate the presence of nuclear receptors in male accessory sex organs for both dihydrotestosterone and estradiol. The differential patterns of3H DHT and3H E2 nuclear uptake suggest differential sensitivities of the individual organs and their tissue compartments for androgens and estrogens.