--- - |2+ Abstract Aim Aging and hypohydration independently attenuate heat dissipation during exercise; however, the interactive effects of these factors remain unclear. We assessed the hypothesis that aging suppresses hypohydration‐induced reductions in whole‐body heat loss during exercise in the heat. Methods On two occasions, eight young (mean [SD]: 24 [4] years) and eight middle‐aged (59 [5] years) men performed 30‐min bouts of light (heat production of 175 W·m−2) and moderate (275 W·m−2) cycling (separated by 15‐min rest) in the heat (40°C, 15% relative humidity) when euhydrated and hypohydrated (~4% reduction in body mass). Heat production and whole‐body net heat exchange (evaporative heat loss + dry heat gain) were measured via indirect and direct calorimetry (respectively) and heat storage was calculated via their temporal summation. Results Net heat exchange was reduced, while heat storage was elevated, in the middle‐aged men during moderate exercise when euhydrated (both P≤0.01). In the young, evaporative heat loss was attenuated in the hypohydrated vs euhydrated condition during light (199±6 vs 211±10 W·m−2; P≤0.01) and moderate (287±15 vs 307±13 W·m−2; P≤0.01) exercise, but was similar in the middle‐aged men, averaging 223±6 and 299±15 W·m−2, respectively, across conditions (both P≥0.32). Heat storage was thereby exacerbated by hypohydration in the young (both P<0.01) but not the middle‐aged (both P≥0.32) during both exercise bouts and, as a result, was similar between groups when hypohydrated (both P≥0.50). Conclusion Hypohydration attenuates heat loss via sweating in young but not middle‐aged men, indicating that aging impairs one's ability to mitigate further sweat‐induced fluid loss during hypohydration. This article is protected by copyright. All rights reserved. - 'Acta Physiologica, Volume 0, Issue ja, -Not available-. '