The cutaneous vasodilation and sweating responses of prepubertal children to heat stress were examined. Seven prepubertal boys (9–11 years old) and 9 young men (20–24 years old) were seated wearing only swimming trunks while the air temperature (T_a) was linearly increased from 28°C to 40°C over 50 min and then maintained at 40°C for an additional 10 min. Skin temperature, cutaneous vascular conductance (CVC), and local sweating rate (m_sw) were measured at multiple sites on the body. The boys had a significantly greater mean surface area-to-mass ratio compared with the young men. The rectal temperature did not change in either group with increasing T_a, although it was significantly higher in the boys. During the first half of the exposure period, when T_a was less than the mean skin temperature (T_sk), the boys had significantly higher CVC on the chest and significantly lower m_sw on the chest and thigh as compared with the young men. During the latter half of the exposure, when heat stress was increased as T_a exceeded T_sk, the boys had significantly higher T_sk, greater CVC on the chest and finger, greater rate of increase in the CVC on the forehead and finger, lower m_sw on the chest and thigh, greater increase in heart rate, and higher thermal sensation. The mean body temperature at the onset of sweating was significantly greater in the boys than in the men. These results suggest that, compared with young men, prepubertal boys manifest greater physiological and perceptual strain under heat stress induced by T_a exceeding T_sk, which is most probably attributable to a combination of lower evaporative heat loss, as evidenced by lower m_sw, and greater heat gain owing to a larger surface area-to-mass ratio. The maturation-related differences in heat loss responses vary according to body site.