The nitric oxide synthases (NOS) are the only heme-containing enzymes that require tetrahydrobiopterin (BH4) as a cofactor. Previous studies indicate that only the fully reduced (i.e., tetrahydro) form of BH4 can support NO synthesis. Here, we characterize pterin-free inducible NOS (iNOS) and iNOS reconstituted with eight different tetrahydro- or dihydropterins to elucidate how changes in pterin side-chain structure and ring oxidation state regulate iNOS. Seven different enzyme properties that are important for catalysis and are thought to involve pterin were studied. Only two properties were found to depend on pterin oxidation state (i.e., they required fully reduced tetrahydropterins) and were independent of side chain structure: NO synthesis and the ability to increase heme-dependent NADPH oxidation in response to substrates. In contrast, five properties were exclusively dependent on pterin side-chain structure or stereochemistry and were independent of pterin oxidation state: pterin binding affinity, and its ability to shift the heme iron to its high-spin state, stabilize the ferrous heme iron coordination structure, support heme iron reduction, and promote iNOS subunit assembly into a dimer. These results clarify how structural versus redox properties of the pterin impact on its multifaceted role in iNOS function. In addition, the data reveal that during NO synthesis all pterin-dependent steps up to and including heme iron reduction can take place independent of the pterin ring oxidation state, indicating that the requirement for fully reduced pterin occurs at a point in catalysis beyond heme iron reduction.