Physical activity is a primary cancer control strategy that has received little attention to date. However, an Increasing number of epidemiological studies have proposed that physical exercise may be beneficial by enhancing anticancer immune system responses. We investigated the effects of acute exercise on changes in nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression. The amounts of NO generated by abdominal macrophages in mice were measured after exercise. Thirty-two mice, which were challenged with thioglycollate broth to activate peritoneal macrophages, were randomly assigned to control, exercise and recovery groups. The mice exercised on a motor-driven treadmill for 3 consecutive days, either moderately (18m/min, 30 min/day, 5％ grade) or severely (18-35m/min, 60 min/day, 5％ grade). The mice were killed immediately after exercise or after 6 hrs of recovery. Nitric oxide was quantified by the Griess assay. The exercised mice showed higher levels of NO generation than those of the control mice, but the intensity of exercise had no significant effect on NO generation. Mice allowed six hours of recovery after exercise showed higher levels of NO generation than that of animals sacrificed immediately after exercise, but there were no significant differences in NO generation with variations in the intensity of exercise. Increased levels of iNOS were found in the exercised groups, and this was greatest in the groups allowed six hours of recovery compared to those groups sacrificed immediately after exercise. The results of this study suggest that acute exercise may enhance an immune response by inducing macrophage-derived NO generation; these results support the epidemiological findings which support the benefits of exercise in the prevention and control of cancer. Further study is needed to determine the physiological significance of these findings, which could be applied to the use of therapeutic exercises to assist in the prevention and control of cancer.