Studies in the streptozotocin rat model for diabetes mellitus suggest that sexual dysfunction in these animals may result from diabetes-induced alterations of the neuroendocrine-reproductive tract axis. Our investigation was performed to better define the effects of diabetes on the neuroendocrine sex accessory organ axis in the male rat. Rats were rendered diabetic, and were either left untreated or treated with insulin, testosterone or both. Diabetes resulted in decreased body and reproductive organ weights, as well as diminished sperm counts and motility and prostatic acid phosphatase. Seminal fructose was increased. A significant decrease in serum LH, FSH and testosterone was noted. Insulin treatment of the diabetic rats restored serum gonadotropins, reproductive organ weight, sperm counts and motility, and seminal fructose to control levels. Prostatic weight and prostatic acid phosphatase levels remained abnormal. Testosterone restored the above mentioned parameters to control levels, with the exception of LH. Treatment with insulin and testosterone had a synergistic effect on spermatogenesis. A GnRH stimulation test demonstrated that pituitaries of diabetic animals had a blunted response, with diminished LH and FSH secretion. In the diabetic animal, there are both pituitary and testicular abnormalities which may be responsible for reproductive dysfunction.