The metabolism of testosterone to 17 beta-hydroxy-5 alpha-androstane-3-one and 5 alpha-androstane-3 alpha, 17 beta-diol by the 800 g supernatant fraction by different parts of the gastrointestinal tract from male rats was investigated. This metabolism tended to be higher in immature than in mature animals. Administration of dexamethasone or long-acting ACTH to immature and mature rats increased testosterone metabolism to 17 beta-hydroxy-5 alpha-androstane-3-one and 5 alpha-androstane-3 alpha, 17 beta-diol by ileum tissue. No such effect could be observed following administration of progesterone, estradiol, prolactin, LH or FSH in mature animals. Development of the gastrointestinal tract from the immature to the mature stage was associated with augmented metabolism of testosterone to 17 beta-hydroxy-5 alpha-androstane-3-one and 5 alpha-androstane-3 alpha, 17 beta-diol in the ileum.