We studied the effects of exercise on the blood fluidity. The subjects were 18 healthy males (21.6±0.8 yr, control : 5, soccer : 9, gymnastics : 4) . We measured the filtration time of whole blood by the Nuclepore filtration method as an index of blood fluidity. The filtration time at rest in the control group, soccer group, and gymnastics group was 59.8±11.9s, 42.5±15.1 s, 28.6±6.7s, respec tively. The filtration time at rest in the soccer group was significantly shorter than that in the control group (p<0.001) . There were no significant differences in filtration time at rest between the control and gymnastics groups, and between the soccer and gymnastics groups. The filtration time after single aerobic exercise was significantly prolonged compared with that before (p<0.001) . The filtration time before and after anaerobic exercise and muscular exercise was not significantly changed. The degree of filtration time prolongation was significantly lower in groups who drank water compared to groups who did not (p<0.001) . The effect of taking a sauna on filtration time was not significant. These results indicates the effect of aerobic exercise on blood fluidity, and the importance of water intake during exercise.