Exercise activates neutrophil burst and this effect is dependent on training status and exercise intensity. In this study, the chronic effect of treadmill exercise on phagocytosis, production of reactive oxygen metabolites and expression of NADPH oxidase components in rat neutrophils was investigated. Neutrophils were obtained by intraperitoneal lavage with PBS. After 11 weeks of training the exercised group showed increased phagocytosis capacity (49%) and production of reactive oxygen metabolites (6.6-fold) when compared with neutrophils from the sedentary group. Exercised had no effect on expression of the membrane components of NADPH oxidase (p22( phox ), gp91( phox )). In contrast, there was an increase of the p47( phox ) mRNA levels (by 126%), the cytosolic component of the enzyme. In addition, exercise increased the protein content of p47( phox ) (by 22%) and of p67( phox ) (by 2.8-fold) in neutrophils. Evidence is then presented that training to moderate exercise increases phagocytosis and production of reactive oxygen metabolites and the expression of p47( phox ) and p67( phox ) in neutrophils. Therefore, moderate exercise might enable neutrophils to respond more efficiently when exposed to pathogens.