The purpose of this study was to determine whether contractile activity associated with running exercise was a prerequisite for neutrophil infiltration into rat tissues. H2O2-dependent myeloperoxidase (MPO) activity for rat (n = 8) liver, heart, and gastrocnemius muscles was assayed after 58 +/- 11 min of running to voluntary exhaustion (25 m/min; 0% grade). MPO activity values measured with 0.6 mM H2O2 were 0.988 +/- 0.331 (SD) U/g (skeletal muscle), 1.563 +/- 0.303 U/g (heart), and 1.652 +/- 0.510 U/g (liver) for control samples, compared with 1.690 +/- 0.321, 3.128 +/- 1.221, and 2.752 +/- 0.437 U/g, respectively, for the exercise group (P < or = 0.05). Kinetic analysis revealed that maximum velocity for all tissues increased as a result of the exercise (P < 0.05). The Michaelis constant (Km) values at rest for all tissues were similar (range 0.53-0.57 mM H2O2; P > or = 0.05). Exercise did not alter the Km values for cardiac and liver samples; however, for skeletal muscle, the Km was 28% lower than control (P < or = 0.05). The results of this study show that, with prolonged running, MPO activity is elevated in most rat tissues and not exclusively in skeletal muscle. Moreover, the metabolic status of the tissues may be an important factor for neutrophil infiltration with exercise and not exclusively the type of muscle contraction, as previously hypothesized.