Aim: To study the effect of diabetes mellitus and insulin treatment on rat penile nitric oxide synthase content.Methods: Male Wistar rats were divided at random into two groups: the Control ( n = 8) and the Diabetic ( n =17). Diabetes mellitus was induced by intraperitoneal injection of streptozotocin. The diabetic animals were then ran domly divided into two subgroups: diabetic rats without insulin treatment ( n = 7) and diabetic rats with insulin treat ment ( n = 10). The neuronal nitric oxide synthase (nNOS) in the penile corpus cavemosum were assayed by immrmo histochemical staining with specific antibody to nNOS and the nNOS-positive nerve fibers were counted semiquantita tively under a high power microscope. Results: The nNOS- positive nerve fibres in diabetic rats with treatment was higher than that in diabetic rats without treatment ( P ＜ 0.05) and lower than that in the controls ( P ＜ 0.01 ). The nNOS-positive nerve fibres in diabetic rat without treatment were also lower than that in the controls ( P ＜ 0.01). Con clusion: In streptozotocin-induced diabetic rats, the nNOS content in the penile corpus cavernosum was significantly decre~ed. Insulin treatment at the dose level employed partially restores the penile nNOS content in these rats.