The uptake and retention of radiolabelled dihydrotestosterone or one of its metabolites by both the male and female reproductive organs were examined in the baboon. 2 male and 2 female baboons were injected intravenously with 1 μg/kg body weight of 3H-dihydrotestosterone and 2 animals, 1 male and 1 female, were injected with both labelled and 100 μg/kg body weight of radio-inert dihydrotestosterone. 1.5 h after the injections, the animals were sacrificed and the uterus, cervix, vagina, uterine tube, labia minora, seminal vesicles, prostate, ductus deferens, and prepuce were removed and treated for autoradiography. The stratified squamous epithelia of the cervix, vagina, labia minora and prepuce demonstrated uptake of label in the germinative, but not in the superficial, cell layers. The columnar cells lining the uterine tube and cervical glands, but not the uterine glands, were labelled. In addition, the nuclei of the cells in the luminal epithelium of the prostate and seminal vesicles contained silver grains. The interstitial cells of the seminal vesicles, prostate and ductus deferens, and the smooth muscle of the prostate and ductus deferens, but not the seminal vesicles, demonstrated an uptake of radioactivit