Testicular function was studied in vivo and in vitro in adult male dy/dy and dy2J/dy2J dystrophic mice. The results demonstrate that testicular function in dy/dy mice is more affected. The basal levels of pituitary hormones measured were normal in dystrophic mice, except for the presence of hyperprolactinemia in dy/dy mice. In dy/dy mice testicular weight was diminished and a deficient transduction of the gonadotropic signal is present in vivo, accompanied by reduced efficiency of 17-hydroxylase and 17-hydroxysteroid dehydrogenase. In dy2J/dy2J mice the signal transduction is normal and the reduction in enzyme efficiency is limited to 17-hydroxysteroid dehydrogenase. The in vitro HCG-induced increases in production of testosterone (T) and estradiol (E2) were reduced in dy/dy/mice, and the data indicate a reduction of enzyme activity rather than in efficiency. In dy21/dy21/mice, HCG-induced T synthesis was increased, HCG-induced E2 synthesis was normal, but basal media E2 levels were reduced, with the in vitro efficiency of aromatase being suppressed under both basal and HCG-stimulated conditions, when compared to their normal littermates.