Diabetes is an endocrine and metabolic disorder often associated with erectile dysfunction and peripheral neuropathy. Among other factors, penile erection is induced by activation of nitric oxide synthase (NOS). Hypothalamic paraventricular nuclei neurons produce NO and project to spinal cord areas implicated in penile reflexes. These nuclei have shown an increase of NOS in streptozotocin-induced diabetic rats. NOS-containing neurons are identical to the populations of neurons selectively stained for NADPH-diaphorase activity. Using this technique, we have evaluated changes of NOS in the lumbar spinal cord of diabetic rats with or without insulin treatment. Positive staining was found in motoneurons, dorsal horn neurons (layer II), neurons surrounding the ependimus (layer X) and neurons at the intermediolateral cell column (ILCC). Diabetic animals showed significant decrease in reactive area and increase of the histochemical reaction in motoneurons from the sexual dimorphic nuclei and in neurons of the ILCC. A marked decrease of the number of reactive neurons was also observed in layer II. Morphologic alterations were observed in neurons of layer X as an increase in the percentage of multipolar neurons and a decrease in the number and length of secondary processes. The alterations observed in these animals were absent in the insulin treated diabetic animals. These results show the plasticity of lumbar spinal cord neurons, suggesting a direct participation of NO synthesis in the physiopathology of the erection dysfunction in diabetes.