Using simple techniques, the neutrophil function, in its phagocytosis and oxidative metabolism stages, was evaluated in horses. This was done before and after moderate exercise at the aerobic-anaerobic threshold (standardized heart rate 150 beats/min and lactate level of 3.07 +/- 0.21 mmol/L). The objective was to determine whether regular training and moderate exercise improved the neutrophil function. A group of 19 horses was used; 11 of these were untrained and the remainder trained for national jumping events. The exercise test consisted of a 5 min trot followed by a 3 min gallop on a long lunge. Blood samples were taken for analysis before, immediately after and 15 min after exercise. The results showed that (a) there is a difference in the internalization of particles (PI, PP and PE) by neutrophils from trained and untrained horses at a single time point during active recovery, and PP is higher in trained horses immediately after exercise; and (b) oxidative metabolism is significantly lower in untrained animals before and 1 min after exercise. The moderate exercise at the aerobic-anaerobic threshold did not have any influence on the peripheral blood neutrophil function of the phagocytosis and oxidative metabolism of particles.