Rectal and skin temperature and sweat rate were measured in eight 11-year-old boys exercising one hour on a bicycle ergometer at each of three different work loads. Rectal temperature rose according to the relative work load and reached a steady state level after a shorter time than had been previously observed in adult subjects. A good relationship was observed between the levelling off value of rectal temperature during work and the relative work load, but the value of rectal temperature at rest just before the start of work affects this steady state value. The average skin temperature was kept fairly constant during exercise except on the heaviest work load (70%), during which it rose about 1 degree C mainly as a result of the rise in skin temperature on the arm, hand and thigh. Temperature increases on skin locations in general were minimal except on the back where the temperature decreased slightly. Sweat rate showed a close relationship to the absolute work load, and in this report the conclusion is supported that the 11-year-old boys regulate their body temperature during exercise at constant work load in the same way as do adult subjects.