Nitric oxide that plays a role in controlling blood pressure showed an anodic voltammetric wave catalyzed by meso-tetra(N-methyl-4-pyridyl) iron(III) pentachloride ([FeIII(TMPyP)]5+) in a phosphate buffer solution (pH 7.4). The current, 10 times larger than the diffusion-controlled current of NO without the iron porphyrin, can be utilized for quantitative determination of NO in aerobic physiological environments. It did not include any oxidation current of nitrite ion due to blocking by the adsorption of the complex. Spectroelectrochemical measurements suggested a formation of iron-nitrosyl complex, which was responsible for the catalytic oxidation of NO. Intermediates of the catalytic oxidation-reduction with [FeIII (TMPyP)]5+ were detected by spectroelectrochemical techniques. They are composed of an oxidation cycle and a reduction cycle, during which NO2- is generated.