Eight older (60–65 years) and six younger (20–25 years) men were exposed to a standard heat stress for 60 min in summer, autumn, winter, and spring. The test consisted of placing the lower legs and feet in a 42°C water bath while sitting in constant environmental conditions (30°C and 45% relative humidity). The increase of rectal temperature (
T
re) was significantly greater (
P < 0.05) in autumn, winter, and spring than in summer for the older group, but significantly greater only in winter than in summer for the younger group (
P < 0.05). The
T
re was greater for the older group in all seasons, but of significance only in autumn and spring (
P < 0.01). There were no significant season-related differences for metabolic heat production (m) and mean skin temperature (
sk) during the heat test in the respective groups, although the m and
sk were lower for the older group in all seasons (
P < 0.01). In the older group total body sweating rate (m
sw) divided by
T
re (total m
sw/
T
re) decreased from summer to winter (
P < 0.02) and did not differ between winter and spring, whereas total m
sw/
T
re in the younger group increased in spring after decreasing from autumn to winter (
P < 0.03). The variations of the value, local sweating rate on the back and thigh divided by
T
re (back m
sw/
T
re and thigh m
sw/
T
re), were similar to those of the total m
sw/
T
re in each group, except for back m
sw/
T
re in the younger group, which did not increase from winter to spring. The total m
sw/
T
re, back m
sw/
T
re and thigh m
sw/
T
re were significantly less for the older group in summer, autumn and spring (
P < 0.05). The range of seasonal variations was significantly less for the older group (
P < 0.001). The results indicated that, compared with younger men in older men, the enhancement of sweating function toward summer occurred later and its reduction toward winter occurred earlier despite a smaller range of seasonal variation and that older men had a somewhat lesser capability to maintain
T
re when challenged by heat stress in all seasons.
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