Derivatives of pterin, an important cofactor for neuronal nitric oxide synthase, markedly changed the kinetics of CO binding to the enzyme-bound heme. In the absence of the substrate, L-Arg, CO binding was dependent on the CO concentration for pterin complexes, whereas in the presence Of L-Arg, it became CO-independent for most pterin complexes. This suggests dual effects of substrate and pterin binding on the kinetics and access of CO to the ligand channel.