On separate days, eight physically active younger (22 +/- 2 years) and eight highly trained middle-aged (45 +/- 4 years) men matched for physical fitness and body composition performed 90 min of semi-recumbent cycling at a constant rate of heat production (290 W) followed by 60 min of seated recovery in either a temperate (T, 30 degrees C), warm (W, 35 degrees C) or hot (H, 40 degrees C) ambient condition. Rectal temperature (T (re)) was measured continuously, while the rate of whole-body heat loss (H (L)), as well as changes in body heat content (H (b)) was measured simultaneously using direct whole-body and indirect calorimetry. No difference in H (L) was observed between age groups for all ambient conditions. Accordingly, the average H (b) during the 90-min exercise was similar for the younger (+193 +/- 52, 212 +/- 82 and +211 +/- 44 kJ for T, W and H, respectively) and middle-aged men (+192 +/- 119, +225 +/- 76 and +217 +/- 130 kJ for T, W and H, respectively). This was paralleled by a similar increase in T (re) of 0.40 +/- 0.20, 0.36 +/- 0.14 and 0.34 +/- 0.23 degrees C for T, W and H, respectively in the younger men and 0.37 +/- 0.23, 0.32 +/- 0.19 and 0.28 +/- 0.14 degrees C for T, W and H, respectively in the middle-aged men. After 60 min of recovery, H (b) was similar for the younger and the middle-aged men, respectively (-45 +/- 52 and -38 +/- 31 kJ for T; -57 +/- 78 and -40 +/- 25 kJ for W; and -32 +/- 71 and 11 +/- 96 kJ for H). End recovery T (re) remained elevated to similar levels in both the younger and middle-aged men, respectively, for each of the ambient conditions (0.24 +/- 019 and 0.18 +/- 0.18 degrees C for T; 0.25 +/- 0.11 and 0.24 +/- 0.14 degrees C for W and 0.33 +/- 0.21 and 0.33 +/- 0.13 degrees C for H). We conclude that highly trained middle-aged men demonstrate a similar capacity for heat dissipation when compared with physically active younger men.