Depletion of glutathione in the substantia nigra is one of the earliest changes observed in Parkinson's disease (PD) and could initiate dopaminergic neuronal degeneration. Nevertheless, experimental glutathione depletion does not result in preferential toxicity to dopaminergic neurons either in vivo or in vitro. Moreover, dopaminergic neurons in culture are preferentially resistant to the toxicity of glutathione depletion, possibly owing to differences in cellular glutathione peroxidase (GPx1) function. However, mesencephalic cultures from GPx1-knockout and wild-type mice were equally susceptible to the toxicity of glutathione depletion, indicating that glutathione also has GPx1-independent functions in neuronal survival. In addition, dopaminergic neurons were more resistant to the toxicity of both glutathione depletion and treatment with peroxides than nondopaminergic neurons regardless of their GPx1 status. To explain this enhanced antioxidant capacity, we hypothesized that tetrahydrobiopterin (BH(4)) may function as an antioxidant in dopaminergic neurons. In agreement, inhibition of BH(4) synthesis increased the susceptibility of dopaminergic neurons to the toxicity of glutathione depletion, whereas increasing BH(4) levels completely protected nondopaminergic neurons against it. Our results suggest that BH(4) functions as a complementary antioxidant to the glutathione/glutathione peroxidase system and that changes in BH(4) levels may contribute to the pathogenesis of PD.