[Objective] Perspiration is almost 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 250g were used. All animals were put in high temperature environment (Wet Bulb Globe Temperature; WBGT: 28°C) through whole experimental period. The rats were divided into four groups randomly; Suitable temperature environment-Exercise-Non water intake (SEN), High temperature environment-Exercise-Non water intake (HEN), High temperature environment-Exercise-Water intake (HEW) and Baseline (BL). In a group of water supply, distilled water was served before and later exercise by sonde forcibly. The blood was collected before or later of exercise and blood and erythrocyte suspension fluidity were measured.[Results] In the HEN, hydroperoxides, blood sodium, lactic acid and adrenaline increased while blood and erythrocyte suspension fluidity were decreased significantly compared with the BL. In addition, the hematocrit did not increase even if water equivalent to 4% of body weight lost it.[Conclusion] We speculate that exercise in high temperature environment decreases blood fluidity. However, the water supply in exercise that might not be sufficiently improve blood fluidity.