The study was conducted to investigate the thermoregulation of young children compared to that of adults. A group of 19 children (ages 9 months-4.5 years), with only 3 children aged 3 years or above, and 16 adults first rested in a thermoneutral room (air temperature 25 degrees C relative humidity 50%, air velocity 0.2 m.s(-1)). They were then exposed to a hot room (air temperature 35 degrees C, relative humidity 70%, air velocity 0.3 m.s(-1)) next door for 30 min, and then returned to the thermoneutral room where they stayed for a further 30 min. The rectal temperature (Tre), skin temperatures (Tsk) at seven sites, heart rate (HR), total sweat rate (Msw,t), local sweat rate (Msw,l) and the Na+ concentration of the sweat were measured. There was no significant difference in Tre between the children and their mothers in the rest phase. However, the Tre of the children increased as soon as they entered the hot room and was significantly higher than during the control period, and than that of the mothers during heat exposure. Mean Tsk, forehead, abdomen and instep Tsk were significantly higher in the children during both the thermoneutral and heat exposure. The Msw,t was significantly higher and Na+ concentrations in the sweat on the back and upperarm were significantly lower for the children during the heat exposure. They had a greater body surface area-to-mass ratio than the mothers by 64%, which indicated that they had advantages for thermal regulation. However, the sweating and Tsk responses of the children were not enough to prevent a rise in body temperature. These results would suggest that the young children had the disadvantage of heating up easily due to their smaller body sizes and there may be maturation-related differences in thermoregulation during the heat exposure between young children and mothers.