Nitric oxide synthases (NOSs)'(L-arginine, NADPH: oxygen oxidoreductases (nitric oxide forming); EC 1.14. 13.39) are unique among eukaryotic enzymes in being dimeric, calmodulin-dependent or calmodulin-containing cytochrome P450-like hemoproteins that combine reductase and oxygenase catalytic domains in one monomer, bear both FAD and FMN, and carry out a 5-electron oxidation of a non-aromatic amino acid (L-arginine) with the aid of tetrahydrobiopterin (1). Eukaryotes as phylogenetically ancient and diverse as Limulus polyphemus (2) and Rhodnius prolixus (3) make NO, and species as distantly related as Drosophila'and man (&lo) have yielded NOS cDNAs. Biosynthesis of nitric oxide (NO) appears to be within the repertoire of most mammalian cell types (ll), provided they are appropriately stimulated. NO has been known for the past 6 years to mediate aspects of macrophage cytotoxicity, regulate blood pressure, and participate in neurotransmission (11, 12), yet a steady stream of reports continues to enlarge NO'S sphere of action. For example, endogenous NO can contribute to destruction of the host's joints (13, 14) and kidneys (14) or to its survival from viral infection (15). Accordingly, regulation of NO biosynthesis has attracted intensive study and is the subject of this review. Other recent surveys deal with the enzymology of NOSs (1, 16) and their physiologic roles (11, 12).