This study was conducted to investigate the thermoregulatory responses of infants during hot and warm exposures. After the subjects first rested in a thermoneutral room (28°C air temperature, 50 % relative humidity), they were then exposed to a hot (35°C, 70%) or warm (31°C, 50%) condition for 30 min and then returned to the thermoneutral room where they stayed for an additional 30 min. The rectal temperature ( T _re), mean skin temperature ( T _sk) and total sweat rate ( M _{sw, t}) were measured and compared to the responses of their mothers. The T _re of infants rose during the hot exposure, even though their M _{sw, t} was twice that of their mothers, and the infants had higher T _sk than the mothers. This result was supported by our previous study (Tsuzuki-Hayakawa et al ., 1995). The rise in T _re of infants was especially rapid during the hot exposure, and was significantly higher at the end of the exposure than that obtained at the end of the warm trial. M _{sw, t} of infants was almost identical at both warm and hot exposures, although the heat loads were clearly different under the conditions of hot and warm. This indicates the thermoregulatory response of infants does not work well and a higher excitability of the sweat centers is evident. Also, infants are more easily dehydrated during warm and hot exposures.