The purpose of this investigation was to compare the thermoregulatory responses during exercise in a hot climate among three age categories. Eight prepubertal (PP), eight young adult (Y) and eight elderly (O) male subjects cycled at an intensity of 50 +/- 1% of their maximum oxygen uptake (V(O2peak)) for 85 min (three 20 min bouts with three 7 min rest periods) in hot and dry conditions (41 +/- 0.67 degrees C, 21 +/- 1% relative humidity). During the exercise-in-heat protocol, rectal temperature (T(re)) skin temperatures (T(sk)), heart rate (HR), V(O2), V(CO2) V(E), RER, sweat rate, and the number of heat activated sweat glands (HASG) were determined. Despite highest and lowest end-exposure T(re) in the Y and O groups, respectively, the rise in rectal temperature (accounting for differences in baseline T(re)) was similar in all age groups. Changes in body heat storage (DeltaS), both absolute and relative to body mass, were highest in the Y and O groups and lowest in the PP group. While end-session as well as changes in mean skin temperature were similar in all three age groups, HR (absolute and percentage of maximum) was significantly lower for the O compared with the PP and Y groups. Total body as well as per body surface sweating rate was significantly lower for the PP group, while body mass-related net metabolic heat production ((M -- W) kg(-1)) and heat gained from the environment were highest in the PP and lowest in the O group. Since mass-related evaporative cooling (E(sk) kg(-1)) and sweating efficiency (E(sk)/M(sw) kg(-1)) were highest in the PP and lowest in the O group, the mass-dependent heat stored in the body (DeltaS kg(-1)) was lowest in the PP (1.87 +/- 0.03 W kg(-1)) and highest in Y and O groups (2.19 +/- 0.08 and 1.97 +/- 0.11 W kg(-1), respectively). Furthermore, it was calculated that while the O group required only 4.1 +/- 0.5 W of heat energy to raise their body core temperature by 1 degrees C, and the Y group needed 6.9 +/- 0.9 W (1 degrees C)(-1), the PP group required as much as 12.3 +/- 0.7 W to heat up their body core temperature by 1 degrees C. These results suggest that in conditions similar to those imposed during this study, age and age-related characteristics affect the overall rate of heat gain as well as the mechanisms through which this heat is being dissipated. While prepubertal boys seem to be the most efficient thermoregulators, the elderly subjects appear to be the least efficient thermoregulators.