Testosterone- 1,2-3H with a specific activity of 46.5 c/mm was injected intramuscularly into 3–4 months old, randomly selected rats castrated 3 days prior to injection. The radioactivity in liver, muscle, blood and the ventral prostate was measured by liquid scintillation counting. A selective uptake of radioactivity was demonstrated in the prostate, with a maximum concentration 1–2 hours following injection. Animals with ureterocolostomy showed the same values, thus eliminating urinary contamination as an explanation for the registered uptake. Simultaneous administration of 20 μg per rat of unlabelled testosterone diminished the uptake in the prostate by an average of 38 % in intact rats, while 500 μg of unlabelled hormone per rat reduced the uptake to background levels in animals castrated 1 day previously. It is suggested that the main reason for earlier failure to demonstrate a selective uptake in the prostate might be the administration of greater amounts of a hormone with too low a specific radioactivity.