The thermoregulatory response to exercise in the heat, especially sweating pattern, differs between children and adults. This study investigated the changes with physical maturation in the thermoregulatory response to exercise (50% V̇O2 max) in the heat (42°C 20% RH) among circum-pubertal boys, using a mixed cross-sectional, longitudinal design. Subjects were initially divided into three groups, based on Tanner (pubic hair) criteria: 16 pre-pubertal (PP, stage I), 15 mid-pubertal (MP, stages II, III, IV), and 5 late-pubertal (LP, stage V). The thermoregulatory response was observed every 6 months for a period of 18 months (4 sessions). Thirty of the 36 boys completed the four sessions. During each session, the exercise task consisted of three 20-min bouts of cycling with 10-min rest periods. Measurements included rectal and skin temperatures and heart rate continuously, V̇O2 at the midpoint of the second bout, sweat collection during each bout, photography of sweat drops after bouts 1 and 2, and whole body sweating rate. During each session, body temperatures tended to be higher among LP relative to the other two groups; however, the rate of increase in body temperatures was similar among groups. Sweating rate per body surface area and per gland were consistently higher among LP compared to PP. This was accompanied by lower sweat lactate concentrations during the initial stages of exercise and lower activated sweat gland population density. Longitudinal observations tended to support cross-sectional findings. It is concluded that physical maturation is characterized by enhanced sweating rate per body surface area and per gland, and that this may be associated with increased sweat gland anaerobic metabolism. © 1992 Wiley-Liss, Inc.