PURPOSE: In an aerobic environment, all biological organisms react with reactive oxygen specise, especially O2 free radicals and hydrogen peroxide. Although acute disorders do not appear, continuous increases of it can lead to aging or cancer. In this study, the acute effects of duroquinone (2,3,5,6-tetramethyl-1,4-benzoquinone, DQ, superoxide radical generator) on corporal smooth muscle was investigated, which causes flavorprotein to produce superoxide via chemical reduction reaction. MATERIAL AND METHOD: Under the same conditions, corporal smooth muscle strips were acquired from 2.0-2.5 kg Male New Zealand White rabbit. Then strips were placed in tension measuring apparatus. After strip was contracted by phenylephrine, an electric field stimulation (EFS)- and various drugs-induced relaxation rate was measured. Relaxation rates by above-mentioned methods were measured again following incubation of DQ or DQ and diethyldithiocarbamate (DETCA, superoxide dismutase inhibitor). The tension percentages were calculated with respect to the control group. RESULTS: The resting tension of the muscle strip was not changed by applying of DQ (0.01, 0.1 and 1 mM). The relaxation rates by EFS and bethanechol were not attenuated after DQ was incubated, but attenuated after DQ and DETCA were incubated. The relaxation rate by ATP was not attenuated after DQ, or DQ and DETCA was incubated. The relaxation rates by sodium nitroprusside was attenuated after DQ only was incubated. CONCLUSIONS: Superoxide seems to eliminate nitric oxide, and thus be an important corporal smooth muscle relaxation inhibitor.