Insulin binding parameters have been measured in testicular membranes of streptozotocin diabetic male rats. Insulin binding decrease was ascribed to the well-known depressing effect of diabetes mellitus on circulating luteinizing hormone (LH). Because both LH and insulin receptors are modulated by pituitary LH and because of their reduction in testes of diabetic rats, we conclude that Leydig cell dysfunction is a secondary disorder associated with this complex metabolic condition.