SummaryThe objective of this study was to determine if strenuous exercise in unconditioned horses impaired function of neutrophils and alveolar macrophages. Flow cytometry was used to assess the effects of a single bout of strenuous exercise on these cells. Bronchoalveolar lavage and venous blood sampling were performed 30 min, 1 and 5 days after exercise. Exercise consisted of 2 min at 2,4 and 6 m/s and then 1 min at 1 m/s incremental speeds until exhaustion, on a level treadmill. Intracellular oxidation of the fluorochrome dye DCFH‐DA was stimulated by PMA and detected by a flow cytometer. Size and granularity of the 2 cell preparations were also measured. The respiratory burst induced in granulocytes was significantly inhibited 30 min after exercise (480 ± 182) compared to that prior to exercise (2359 ± 456). Inhibition at 1 and 5 days after exercise was less than at 30 min and did not differ significantly from either control values. No change in size or granularity was seen as a result of exercise. The alveolar macrophage population remained unchanged in size, granularity and fluorescent responses following exercise. Several cell samples taken before exercise exhibited peripheral blood eosinophilia. In these samples, granulocyte size was markedly reduced and DCFH‐DA oxidation was depressed; viability did not alter. This study suggests that strenuous exercise impairs the respiratory burst of granulocytes for several days. Other factors associated with peripheral eosinophilia also have suppressive effects on granulo