The immune response to acute exercise after long-term training was evaluated in rats. After 10 wk of exercise training on a drum exerciser, both sedentary control and trained groups ran with the intensity of 70% of peak oxygen consumption (&OV0312;O2peak) for 10 min immediately before sacrifice. The mitogenic activity of spleen lymphocytes in trained group to staphylococcal enterotoxin B, a polyclonal T cell activator, decreased as compared to the sedentary control following acute exercise. However, proliferative response to lipopolysaccharide, a B cell mitogen, was augmented. Furthermore, the interleukin-2 production was reduced in the trained group. The lymphocyte subpopulations in the spleen and the peripheral blood lymphocytes were analyzed by flow cytometry. Although the tendency of changes in some populations was observed in trained vs untrained groups, no statistically significant difference was manifested. The serum levels of both norepinephrine and epinephrine increased immediately after acute exercise. The increase in serum lactate concentration was observed following acute exercise in sedentary control, but less prominent in the trained group.