Introduction: Heatstroke is a life-threatening illness caused by an elevation of the body temperature due to exposure to high temperature and/or humidity environments, and its incidence has been increasing in the summer in Japan. It is considered to be important to drink water to prevent the development of heatstroke. OS-1 is an oral rehydration solution which was designed to alleviate dehydration due to various conditions, such as cholera, and contains electrolyte ions such as sodium (50 mEq/l), potassium (20 mEq/l), chloride (50 mEq/l), magnesium (2 mEq/l), sulfate (2 mEq/l) and phosphate (2 mmol/l), as well as lactate (31 mEq/l) and glucose (18 g/l). Although drinking OS-1 is in the summer time, the effects of OS-1 supplementation on heatstroke have not been examined in detail. We produced a non-lethal experimental heatstroke model in mice, and then determined their core body temperature after supplying either pure water or OS-1. Methods: Experimental heatstroke was produced in a moisture chamber. The mean conditions of the chamber were an ambient temperature of 41 ± 0.5 ºC and relative humidity at 70 - 99% during the heat-exposure (HE). During the experiment, the ambient temperature, relative humidity and Wet-Bulb Globe Temperature (WGBT) were monitored every 1 minute. Male C57BL/6J mice had a thermometer implanted into the abdominal cavity three weeks before the experiments, and were maintained at a normal temperature (ambient temperature 24 ± 1ºC and relative humidity 20–40%) with free access to food and water. The animals were randomly divided into three groups: a no supplement group (control, n=6), 30 mg/kg water supplement group (Water, n=6) and 30 mg/kg OS-1 supplement group (OS-1, n=6) and were subsequently not allowed access to any drink for three hours. Then, the animals were subjected to HE for 60 min in the moisture chamber. The supplement was administered twice orally three hours after dehydration (before HE) and immediately after HE. The animals were then returned to a normal temperature environment and maintained for a further seven days with free access to food and water. It was previously reported that the core body temperature of mice rapidly elevated and reached the maximal temperature after the onset of HE, then it decreased immediately and severe hypothermia was induced after the HE. It took several hours to recover to the normal circadian rhythms. The level of hypothermia was correlated with the severity of HE in mice. In our experiments, the core body temperature was recorded every five minutes from one day prior to HE to five days post-HE in all the mice. The circadian rhythms of the core body temperatures before HE were recorded. The maximum core body temperature during the HE, minimum core body temperatures after the HE and the time to recover to normal circadian rhythm post-HE were compared among the groups. Results: Actual mean condition of the moisture chamber during HE was ambient temperature (39.0 ± 0.16 ºC), relative humidity (93.7 ± 0.11 %) and WGBT (41.6 ± 0.18 ºC). The control group was account for maximal temperature at 32 ± 2.5 min after onset of HE and measured at 41.1 ± 0.1 ºC. The maximal temperature in water and OS-1 groups was 40.9 ± 0.4 ºC and 39.7 ± 0.17 ºC, respectively. Maximal temperature in OS-1 group was significantly lower than that in control (p<0.05). Minimal body temperature which was account for 236 ± 33 min after onset of HE, was no difference between three groups (Control: 31.7 ± 1.45 ºC, water: 31.3kelou ºC, and OS-1: 33.3 ± 0.44 ºC). However, while the circadian rhythm of body temperature was lost during the HE, recovery of the rhythm in OS-1 group (495 ± 48 minutes) was significantly shorter than that in water group (961 ± 138 minutes, p<0.05) and was tend to shorter than control (881 ± 120 minutes, p = 0.056). Conclusions: These results suggest that OS-1 supplementation before and after HE decreased the dysregulation of body temperature, likely due to the efficient rehydration.