We sought the mechanisms in the maturation-related change of skin blood flow to heat stress. Eight prepubertal boys (7-11 years) and 11 young men (21-25 years) were exposed to a mild passive heating [by placing the lower legs and feet in a 42 degrees C water bath for 60 min while sitting in a neutral air condition (25 degrees C, 45% relative humidity)]. No age-related differences were observed for the increase in rectal temperature [0.61 +/- 0.05 (SEM) vs. 0.62 +/- 0.09 degrees C in the boys and men at the end of the passive heating, respectively], mean skin temperature (34.51 +/- 0.28 vs. 34.81 +/- 0.27 degrees C) or metabolic heat production (68 +/- 4 vs. 60 +/- 3 W m-2) during the passive heating. During the heating age-related differences in skin blood flow by laser Doppler flowmetry (LDF) and local sweating rate (m(sw)) varied by site; the boys had greater LDF on the chest and back, similar LDF on the forearm and thigh, lower m(sw) on the chest and thigh, similar (m)sw on the back, and greater m(sw) on the forearm, compared with the men. The relationship between LDF and m(sw) during the heat exposure was divided into three temporal phases: (a) an increase of LDF without an increase in m(sw), (b) an increase of m(sw)without the secondary increase of LDF, (c) a proportional increase of LDF and m(sw) increase of LDF in phase (a) and the slopes of the regression lines between the LDF and m(sw) in phase (c) were significantly greater on the chest and back for the boys (P < 0.05), compared with the men, but not on the forearm and thigh. These results suggest that the greater LDF observed on the trunk in the boys may be owing to a greater withdrawal of vasoconstrictor tone and a greater active vasodilation. Regional differences may exist in the maturation-related alterations in vasoconstriction and vasodilation.