The present study was performed to further clarify the possible role played by insulin deficiency on the steroidogenic capacity of the rat testis. Sprague-Dawley rats weighing 250-300 g were used in all experiments. Diabetes was induced by i.p. injection (40 mg/kg b.w.) of streptozotocin and was monitored at 2-day intervals by measuring body weight and serum glucose, glucosuria and ketonuria levels. The effect of insulin therapy on pituitary LH content and plasma LH concentrations, as well as on the cyclic AMP level in interstitial cell incubation medium and plasma testosterone concentrations, was measured 30 days after the induction of diabetes by radioimmunoassay. Streptozotocin-induced diabetes resulted in significantly reduced pituitary LH (16%, P less than 0.025) and plasma LH (34%, P less than 0.02); insulin treatment completely restored these levels. Similarly, the cyclic AMP content of interstitial cell incubation medium and the plasma testosterone concentrations were dramatically decreased in the diabetic state (50%, P less than 0.005 and 63%, P less than 0.025, respectively) and combined treatment with insulin plus hCG appeared slightly more effective than treatment with either of these hormones alone, suggesting a possible synergistic action. It is concluded that decreased testicular steroidogenesis in the diabetic rat may represent, at least in part, a direct consequence of insulin deficiency at the hypothalamic and/or pituitary levels. However, our findings would also be consistent with other reports suggesting that insulin may play a direct role in the rat testis.