Twenty endurance-trained athletes (five male speed-skaters, eleven male and four female cross-country skiers, 16-18 years) ran on a treadmill by a protocol of incremental graded increase in workload until exhaustion during an endurance training period in off-season summer. Immediately after exercise, all developed peripheral leukocytosis (1.9 times; p<0.01) due mainly to lymphocytosis (2.6 times; p<0.01) with a predominant effect on large granular lymphocyte (natural killer cell) count (5.9 times ; p<0.01) . Monocyte count was also enhanced 2.3 times (p<0.01) . These increases were transitory and returned to the pre-exercise levels 1 h later. Peripheral neutrophilia was also observed by 43% (p<0.01) immediately after exercise and remained elevated by 25% (p<0.01) 1 h after exercise, but a shift to the left did not take place. The capacity of isolated neutrophils to produce reactive oxygen species was assessed by luminol-dependent chemiluminescence which detects mainly myeloperoxidase (MPO) -mediated formation of such hyperreactive oxidants as HOCl. The maximum intensity of chemiluminescence (peak height) upon stimulation with opsonized zymosan was significantly enhanced following exercise (p<0.05) . Similar results were obtained when phorbol myristate acetate was employed as nonphagocytic soluble stimulus (p<0.01), suggesting that the capacity of neutrophils to degranulate MPO rather than phagocytosis was enhanced following exercise. In addition, the enhancements of chemiluminescence were positively correlated with the increase in segmented neutrophil count. These data indicate that maximal exercise not only mobilized mature neutrophils from the marginated pool into the circulation, but also augmented their capacity to generate reactive oxygen species of higher reactivity.