The authors evaluated whether supplementation of tetrahydrobiopterin (BH4) or the BH4 precursor in vascular smooth muscle cells (VSMC) that are deficient in de novo BH4 production affected the ability of these cells to synthesize nitric oxide (NO). GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme for the de novo synthetic pathway of BH4. The selective GTPCH inhibitor 2, 4-diamino-6-hydroxypyrimidine (DAHP) restricts the de novo synthesis of BH4 in VSMC. Thus, treatment with DAHP and cytokines should lead to the intracellular accumulation of inactive inducible nitric oxide synthase (iNOS) apoenzyme (relative to BH4), which can be reactivated upon the repletion of BH4. Using such VSMC deficient in de novo BH4, the authors evaluated their ability to synthesize NO by supplementing with BH4 or the BH4 precursor sepiapterin or dihydrobiopterin. Adding BH4 to VSMC increased NO production in a concentration-dependent manner. Sepiapterin (SEP) or 7, 8-dihydrobiopterin (BH2) also dose-dependently induced NO generation. Nitric oxide was produced in the order SEP> BH2≫ BH4 at half-maximal concentrations for stimulation of 0.05, 0.1, and 1 μmol/L, respectively. Addition of BH4 or SEP substantially induced iNOS enzyme activity, which was quantified as the formation of [3 H] L-citrulline from [3 H] L-arginine. SEP or BH2 in the presence of methotrexate, a selective inhibitor of dihydrofolate reductase, no longer induced NO production. In contrast, although the amount of NO induced by supplemented BH4 was substantially depressed, higher concentrations of BH4, but not SEP or BH2, significantly caused NO production even in the presence of methotrexate. Thus, BH4 produced from BH2 is largely responsible for NO production in VSMC that are deficient in de novo BH4. NOS would preferentially use BH4 that is regenerated from BH2 via dihydrofolate reductase, rather than BH4 or BH2 obtained from the cytosolic milieu.