Subsaturating levels of tetrahydrobiopterin (BH₄), an essential cofactor for nitric oxide synthase (NOS), can lead to endothelial dysfunction as a result of decreased production of nitric oxide. Furthermore, insufficient BH₄ can also result in NOS-uncoupled production of reactive oxygen intermediates, such as superoxide anion and hydrogen peroxide. Nitric oxide and superoxide react rapidly to form peroxynitrite, which may be the reactive species responsible for many of the toxic effects of nitric oxide. Here we show that BH₄ is a primary target for peroxynitrite-catalyzed oxidation because at pH 7.4, physiologically relevant concentrations of BH₄ are oxidized rapidly by low concentrations of peroxynitrite. Peroxynitrite oxidizes BH₄ to quinonoid 5,6-dihydrobiopterin and a large proportion of the quinonoid isomer readily loses its side chain to form 7,8-dihydropterin which is not a cofactor for nitric oxide synthase. Thus, abnormally low levels of BH₄ can promote a cycle of its own destruction mediated by nitric oxide synthase-dependent formation of peroxynitrite. This mechanism might contribute to vascular endothelial dysfunction induced by oxidative stress.