[Objective] Physical exercises raise more or less body temperature. A body temperature is regulated constantly generally by homeostasis mechanism. Perspiration is only heat radiation mechanism under high temperature environments. And sudoriferous water is supplied from blood. Blood flow is determined by blood fluidity, blood volume and the cardiovascular system. It was reported that strong stress decreased blood fluidity.In this experiment, we investigated the relation between blood fluidity and water supply in rats loaded with forced exercise in high temperature environment.[Methods] SPF male Wistar rats weighing 150 g were used. All animals were put in high temperature environment (Wet Bulb Globe Temperature; WBGT: 28°C) through whole experimental period. In a group of water supply, distilled water was served before and later exercise by sonde forcibly. The rats were divided into five groups randomly; Rest-Non water intake (RN), Rest-Water intake (RW), Exercise-Non water intake (EN), Exercise-Water intake (EW) and Baseline (B). The blood was collected before or later of exercise and blood fluidity or platelet aggregation was measured.[Results] In the EN, platelet aggregation, lactic acid and corticosterone increased while blood fluidity were decreased significantly compared with the RN, RW and EW. In addition, the hematocrit did not increase even if water equivalent to 8 % of body weight lost it.[Conclusion] We speculate that exercise in high temperature environment decreases blood fluidity. However, the water supply that does not completely make up for quantity of depletion in exercise may improve blood fluidity.