Androgen administration to immature male rats resulted in the neosynthesis of thymidine kinase (TK) molecules in prostates. Induction was maximum 36–48 hours after hormone administration. It was specifically due to androgens and was dose‐dependent. Separate measurements of TK isoenzyme (fetal and adult types TK) activities showed that in immature animals both activities were stimulated by androgens to different extents. In adult animals, only the isoenzyme of the fetal type was concerned. Results obtained in immature and adult castrated rats showed that the increase in activity of the fetal‐type enzyme was much more important in immature than in adult animals.

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