1. Alteration in the flow of blood to the penis is thought to be the most frequent organic cause of erectile dysfunction or impotence. However, information concerning the penile small arteries (helicine arteries) which control blood flow between the arterial systemic circulation and the cavernous sinusoids is scarce. Therefore, the purpose of the present study was to investigate the involvement of nitric oxide, which is considered to be the main neurotransmitter in corpus cavernosum, in both the non-adrenergic non-cholinergic inhibitory neurotransmission and the endothelium-dependent responses in human penile small arteries. 2. Penile small arteries (lumen diameter 200-700 microns), which were branches of the human deep penile arteries obtained either from patients undergoing penile surgery or from organ donors, were mounted in microvascular myographs for isometric tension recording and electrical field stimulation was performed in the presence of guanethidine and atropine to block adrenergic neurotransmission and muscarinic receptors, respectively. 3. In phenylephrine-contracted penile small arteries, electrical field stimulation (0.5-32 Hz) induced frequency-dependent relaxations of both endothelium-intact and -denuded preparations. The inhibitor of nitric oxide synthase, NG-nitro-L-arginine (3 x 10(-5) mol/l), abolished the relaxations at the lowest frequencies, while slow-developing relaxations were still observed at high frequencies (16 and 32 Hz). The inhibitory effect of NG-nitro-L-arginine was reversed in the presence of L-arginine (3 x 10(-3) mol/l). Tetrodotoxin totally abolished the relaxations to electrical field stimulation. In contracted small penile arteries in the presence of NG-nitro-L-arginine, the nitrovasodilator sodium nitroprusside induced potent relaxations. 4. The endothelium-dependnet vasodilator acetylcholine induced relaxations of penile small arteries, which were only partially reduced in the presence of NG-nitro-L-arginine. In contrast, the relaxations to acetylcholine of trabecular corpus cavernosum preparations were almost abolished in the presence of NG-nitro-L-arginine. 5. The present study suggests that relaxations of human intracavernosal small penile arteries induced by non-adrenergic non-cholinergic nerve stimulation partially involve nitric oxide and also another inhibitory transmitter causing relaxations resistant to nitric oxide synthase blockade. In addition, endothelium-dependnet relaxations in human small penile arteries are mediated by both nitric oxide and a factor resistant to NG-nitro-L-arginine.