[7α-3H]Progesterone was administered to two groups of mice. The localization of radioactivity in tissues was examined by autoradiography and by liquid scintillation counting. Particular attention was paid to the distribution of radioactivity in the liver and biliary system. The greatest amount of administered steroid was taken up by the liver, rapidly secreted into the biliary system and excreted into the gastrointestinal tract. Other tissues, including the uterus, did not take up any appreciable amount of radioactivity, and the autoradiographs showed that radioactivity was not associated with any particular region of hepatic and other cells. The biological half-life of radioactivity in the blood was 11 min in animals given labelled progesterone intravenously.