Testosterone and 5 α‐dihydrotestosterone normally induce alcohol dehydrogenase (ADH) production in the mouse kidney. The following results suggest that the X‐linked testicular feminization (T fm) mutation in the mouse is anon‐induciblemutation of a repressive regulatory gene locus. Eleven affected XTfmY mice and 3 XTfmX+heterozygous females, as well as their normal sibs, were challenged with 10 mg of either testosterone or 5α‐dihydrotestosterone and killed 5 days later. Kidney ADH of normal sibs were in the fully induced state, while no detectable kidney ADH activity was found in XTemY. In accordance with the X‐inactivation hypothesis, individual kidneys of heterozygous females showed extremely variable degrees of inducibility of