Objective: To assess the correlation between neuronal nitric oxide synthase (nNOS) expression in the brain, lumbosacral spinal cord, sympathetic trunk, prostate and penis, and erectile dysfunction (ED) associated with diabetes mellitus (DM). Methods: DM was induced in adult male Sprague-Dawley rats by injecting streptozotocin. After 1, 2, 3 and 4 months, apomorphine was injected to test the penile reflex. The animals were then sacrificed, and the corpora cavernosa were isolated for a diastolic function test. In addition, the cortex of the brain, lumbosacral spinal cord, thoracic and lumbar sympathetic trunk, and root of the penis and prostate of each animal were subjected to a nNOS catalytic activity assay. Results: The times of penile reflex decreased gradually with the prolongation of DM. Acetylcholine-induced corpus cavernosum relaxation also markedly decreased in DM rats. On the other hand, in DM rats nNOS expression was elevated in the temporal cortex, hippocampus and hypothalamus, but was reduced in the spinal cord, sympathetic trunk and prostate. Statistics showed that nNOS expression in the brain correlated negatively with penis reflex and corpus cavernosum relaxation, while nNOS expression in the spinal cord, sympathetic trunk, penis and prostate had a positive correlation with penis reflex and corpus cavernosum relaxation. Conclusion: The expression of nNOS in the central and peripheral nervous systems correlated with diabetic ED and was a cause of diabetic ED.