Aim: To study the effect of intracavemous sodium nitropmsside (SNP), a nitric oxide (NO) donor, on penile erec-tion. Methods: Forty-two patients with erectile dysfunction (ED) were randomly assigned to receive SNP 300μgor the control drags (papaverine 30 mg + phentolamine 1 mg) intracavemously crosswise one week apart. The penilelength, circumference and hardness after the administration of the experimental and control drags were assessed andcompared statistically. Results: (1) There was no significant difference between the changes in penile length andcircumference in the two occasions; (2) In 25 SNP and 28 control cases, the hardness of the penis was scored above100 as evaluated by the Vimg method (P > 0.05); (3) The duration of erection in the controls was longer than that inthe SNP, but there were three priapism in the controls and not a single one in the SNP; (4) there was no apparentchange in the heart rate and blood pressure in both occasions; other side effects were minimal except slight local Aim: To study the effect of intracavemous sodium nitropmsside (SNP), a nitric oxide (NO) donor, on penile erec- tion. Methods: Forty-two patients with erectile dysfunction (ED) were randomly assigned to receive SNP 300 μgor the control drags The penilelength, circumference and hardness after the administration of the experimental and control drags were evaluated andcompared statistically. (1) There was no significant difference between the changes in penile (papaverine 30 mg + phentolamine 1 mg) intracavemously crosswise one week apart. length and circumferences in the two occasions; (2) In 25 SNP and 28 control cases, the hardness of the penis was scored above 100 as evaluated by the Vimg method (P> 0.05); (3) The duration of erection in the controls was longer than that inthe SNP, but there were three priapism in the controls and not a single one in the SNP; (4) there was no apparentchange in the heart rate and blood pressure in both occasions; other side effects were minimal ex cept slight local