The effects of experimentally induced diabetes on the production, metabolism and transport of androgens in the rat testis were examined. Streptozotocin diabetes diminishes the serum and testicular levels of testosterone and increases the enzymic activity that converts testosterone to 5 .alpha.-reduced metabolites, in the seminiferous tubules. The concentration of the androgen binding protein (ABP) in caput epididymis was also reduced in diabetic animals. Although the changes could be attributed to differences in gonadotropin production, these phenomena appear to be related to insulin deficiency, since the testes from diabetic rats treated for 2 wk with insulin showed parameters that were restored to the levels observed in normal animals. Testicular function may be regulated by the action of insulin.