Several studies have suggested that nongonadotrophic pituitary secretions influence growth and function of the rat ventral prostate; the basis for this hypophysial effect is unclear. In order to investigate whether hypophysectomy would alter the previously demonstrated special avidity of rat ventral pro-static tissue for testosterone from that seen in intact animals, ventral prostate uptake and discharge of radioactivity after intravenous injection of testosterone-4-C14 was studied in intact, castrate and hypophy-sectomized adult rats. Following intravenous injection of testosterone-4-C14 to normal rats, only the excretory organs, the liver, kidney and lung and the adrenal and ventral prostate yielded significantly high levels of radioactivity. Timed studies revealed rapid decrease in radioactivity in all tissues over the first 30-60 min. in control rats, whereas decline in radioactivity was less in hypophysectomized animals, presumably because of impaired hepatic excretion of the isotpe resulting in higher levels of radioactivity in all tissue studied. Because of varying blood levels of radioactivity between the groups studied, relative ventral prostate affinity for the labeled hormone was expressed as the ratio of ventral prostate activity to that of rectus abdominus muscle activity, a relatively non-androgen-dependent tissue. In this manner it was shown that accumulation of radioactivity in intact and castrate rats was significantly greater than that seen in ventral prostates of hypophysectomized animals at 30 and 60 min. after intravenous injection of the labeled androgen. It is suggested that the impaired localization of testosterone in the prostate of the hypophysectomized animals, shown in these studies, supports the hypothesis that certain, as yet unidentified, nongonadotrophic pituitary hormones exert their actions on prostatic growth and responsiveness to testosterone by playing a permissive role in enhancing androgen localization in the specific target organ.