Vascular endothelial cells and neutrophils synthesize and release potent vasodilatatory factors, i.e. endothelium-derived relaxing factors (EDRF) and neutrophil-derived relaxing factors (NDRF). One EDRF has been identified as nitric oxide (NO) derived from arginine. We studied the synthesis and release of NO from human neutrophils stimulated with the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine, platelet activating factor or leukotriene B4. The formation and release of NO was enhanced several-fold in the presence of superoxide dismutase, probably by inhibiting superoxide-induced breakdown of NO. The formation and release of NO but not the formation of superoxide anions was decreased in neutrophils pretreated with L-canavanine, an inhibitor of arginine-utilizing enzymes. Our data suggest that at least one NDRF is identical with NO or another labile NO containing compound derived from arginine.