Nitric oxide provokes vasodilation and inhibits platelet aggregation. We examined the effect of nitric oxide on superoxide anion production by three sources: activated intact neutrophils, xanthine oxidase/hypoxanthine, and the NADPH oxidase. Nitric oxide significantly inhibited the generation of superoxide anion by neutrophils exposed to either FMLP (10(-7)M) or PMA (150 ng/ml) (IC50 = 30 microM). To determine whether the effect of nitric oxide on the respiratory burst was due to simple scavenging of O2+, kinetic studies that compared effects on neutrophils and the cell-free xanthine oxidase system were performed. Nitric oxide inhibited O2+ produced by xanthine oxidase only when added simultaneously with substrate, consistent with the short half-life of NO in oxygenated solution. In contrast, the addition of nitric oxide to neutrophils 20 min before FMLP resulted in the inhibition of O2+ production, which suggests formation of a stable intermediate. The effect of nitric oxide on the cell-free NADPH oxidase superoxide-generating system was also examined: The addition of NO before arachidonate activation (t = -6 min) significantly inhibited superoxide anion production. Nitric oxide did not inhibit O2+ when added at NADPH initiation (t = 0). Treatment of the membrane but not cytosolic component of the oxidase was sufficient to inhibit O2+ generation. The data suggest that nitric oxide inhibits neutrophil O2+ production via direct effects on membrane components of the NADPH oxidase. This action must occur before the assembly of the activated complex.