Tetrahydrobiopterin (H(4)Bip) is a cofactor for several key enzymes, including NO synthases and aromatic amino acid hydroxylases (AAHs). Normal functioning of the H(4)Bip regeneration cycle is extremely important for the work of AAHs. Oxidized pterins may accumulate if the H(4)Bip regeneration cycle is disrupted or if H(4)Bip autoxidation occurs. These oxidized pterins can photosensitize the production of singlet molecular oxygen O-1(2) and thus cause oxidative stress. In this context, we studied the photooxidation of H(4)Bip in phosphate buffer at pH 7.2. We found that UV irradiation of H(4)Bip affected its oxidation rate (quantum yield phi(300) = (2.7 +/- 0.4) x 10(-3)). The effect of UV irradiation at =350nm on H(4)Bip oxidation was stronger, especially in the presence of biopterin (Bip) (phi(350) = (9.7 +/- 1.5) x 10(-3)). We showed that the rate of H(4)Bip oxidation linearly depends on Bip concentration. Experiments with KI, a selective quencher of triplet pterins at micromolar concentrations, demonstrated that the oxidation is sensitized by the triplet state biopterin (3)Bip. Apparently, electron transfer sensitization (Type-I mechanism) is dominant. Energy transfer (Type-II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H(4)Bip photooxidation and their biological meaning are discussed.