The binding of tetrahydropteridines with 6-di- and trihydroxypropyl side chains to recombinant rat neuronal nitric oxide (NO) synthase (EC 1.14.13.39) was determined by competition with 6R-[3'-3H]-5,6,7,8-tetrahydro-L-erythro-biopterin (6R-[3'-3H]H4biopterin). Although all but one of the derivatives exhibited only poor affinities (Ki 50 microM), the 4-amino analogue of 6R-H4 biopterin was a potent antagonist of 6R-H4 biopterin binding (Ki 13.2 nM). The 4-amino analogue of 6R-H4 biopterin inhibited NO synthase stimulation by the natural cofactor 6R-H4 biopterin with an IC50 of 1 microM without affecting the basal activity observed in the absence of added 6R-H4 biopterin. Because the 4-amino analogue of 6R-H4biopterin also inhibited dihydropteridine reductase (EC 1.6.99.7; IC50 20 microM), our results support the hypothesis that redox cycling of H4 biopterin might be required for the NO synthase reaction.