[6,7-3H] 17β-Oestradiol with a specific activity of 42.4 Ci/mmole was injected intramuscularly into three to four month old male rats, castrated three days previously. The radioactivity in liver, skeletal muscle, blood, the anterior pituitary gland, the seminal vesicles and in the different prostatic lobes was measured by liquid scintillation counting at different intervals after the administration. A high and prolonged uptake of radioactivity was found in the anterior pituitary gland. The uptake by the accessory sex organs was much lower, but significantly higher than that by skeletal muscle. The uptake by the prostate and the seminal vesicles in castrated animals was similar to that in non-castrated animals. The pattern of radioactivity uptake in the anterior pituitary gland of castrated male rats given [3H] testosterone was distinctly different from that after administration of [3H] 17β-oestradiol. There was a rapid elimination of radioactivity from the adenohypophysis after the administration of [3H] testosterone.