The inactivation of dihydroorotate dehydrogenase was studied by irradiating at selective wavelengths, namely 280 nm and 450 nm, using a steady-state Xenon lamp as a light source. The activity of the enzyme decreased exponentially as a function of the absorbed dose under both aerated and deaerated conditions. The inactivation in deaerated conditions is higher than that in aerated conditions when enzyme solutions were irradiated at 450 nm, whereas inactivation under aerated conditions was slightly higher compared with that in Ar-saturated condition when irradiated at 280 nm. No change in fluorescence spectral shape was observed, however, intensity of emission maximum was found to decrease, except in one case. The fluorescence intensity of flavin was found to increase with absorbed dose when the enzyme was irradiated at 280 nm in aerated solution. Changes in the kinetic parameters (Michaelis-Menten constant, Km, and maximal velocity, Vmax) due to irradiation at 280 nm suggests that the substrate-binding site is modified in deaerated conditions but not in aerated conditions.