This study examined effects of hyperoxia on thermoregulatory responses. Eight healthy male students (23.5±1.8 yrs) were involved in this study. They immersed their legs in a hot water bath (42°C) for 60 minutes in a climate chamber. The conditions of oxygen concentration of a chamber were set at 21% (control), 25% (25%O₂), and 30% (30%O₂). Ambient temperature and relative humidity was maintained at 25°C and 50% in every condition, respectively. Measurements included rectal temperature (Tre), skin temperature at 7 sites, laser Doppler flowmeter (LDF) on the back and forearm as an index of skin blood flow, heart rate, local sweat rate (Msw) on the back and forearm, and total body weight loss (BWL). Increases of Tre at 25%O₂ and 30%O₂ tended to be lower during the immersion than in the control. Mean skin temperature (Tsk) of the control increased gradually after the onset of sweating, while the Tsks at 25%O₂ and 30%O₂ maintained a constant level during sweating. LDFs on the forearm at 25%O₂ and 30%O₂ showed lower increases compared with the control. No significant differences in Msw on the back and the forearm and BWL were seen among the conditions. These results suggested that hyperoxia could not affect sweating responses but elicit an inhibitory effect on thermoregulatory skin blood flow.