The purpose of this study was to examine reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity, - a generated superoxide - of neutrophils in human peripheral blood after maximal exercise. Ten healthy male college students (20.2 ± 0.4 yr, 170.5 ± 1.3 cm, 62.8 ± 1.9 kg) participated after giving written informed consent. They performed an incremental exercise to volitional exhaustion using a bicycle ergometer. Peripheral blood was collected before exercise (Pre), just after exercise (Post) and 1-hour after exercise (Post-1h). Phorbol 12-myristate 13-acetate (PMA)-stimulated and opsonaized zymosan (OZ)-stimulated superoxide-generating activity of neutrophils was measured by the cytochrome c reduction assay. NADPH oxidase activity was measured by a cell-free system. NADPH oxidase activity significantly decreased in Post-1h compared with Pre and Post. A similar tendency was seen in PMA-stimulated activity, but not in OZ-stimulated activity. A strong positive relationship between NADPH oxidase activity and PMA-stimulated activity was found in Pre and this relationship attenuated after exercise. NADPH oxidase activity was not related to OZ-stimulated activity at any time points. We concluded that NADPH oxidase activity decreased after exhaustive maximal exercise in human peripheral neutrophils, and suggest that PMA-stimulated activity, relatively - speaking, reflects NADPH oxidase activity; but OZ-stimulated activity is independent of NADPH oxidase activity.