Diabetes mellitus is associated with impotence in animal models and patients. Raised reactive oxygen species contribute to diabetic neurovascular deficits, which are amenable to antioxidant treatment. Our aim was to examine the effects of streptozotocin-induced diabetes in rats and long-term treatment with the antioxidant, alpha-lipoic acid, on responses of an in vitro corpus cavernosum preparation. Diabetes duration was 8 weeks and preventive and reversal (4 weeks untreated diabetes, 4 weeks of treatment) studies were done. Four and 8 weeks of diabetes caused an about 41% reduction in endothelium-dependent nitric oxide mediated relaxation to acetylcholine in phenylephrine-precontracted cavernosum. This deficit was prevented (93.9+/-7.1%) by treatment with alpha-lipoic acid; reversal studies showed 64.9+/-19.5% correction. Neither diabetes nor treatment with alpha-lipoic acid altered endothelium-independent relaxation to the nitric oxide donor, sodium nitroprusside. Stimulation of corpus cavernosum autonomic innervation caused noradrenergic-mediated contractions that were unaffected by diabetes or alpha-lipoic acid. Non-adrenergic, non-cholinergic nerve responses, largely dependent on nitric oxide, were seen after phenylephrine precontraction in the presence of atropine and guanethidine. Non-adrenergic, non-cholinergic stimulation caused frequency dependent relaxation to a maximum of about 40%. Diabetes reduced this to about 25%, however with preventive alpha-lipoic acid treatment, non-adrenergic, noncholinergic relaxation was in the nondiabetic range. In the reversal alpha-lipoic acid treated diabetic group, its deficit was corrected by 52.1+/-14.6%. Thus, diabetes reduces endothelium and non-adrenergic, noncholinergic nerve nitric oxide-mediated relaxation of corpus cavernosum smooth muscle, which is likely to be the organic base for impotence. Prevention and partial correction by alpha-lipoic acid emphasises the importance of reactive oxygen species and suggests a potential therapeutic approach.