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This study tested the hypothesis that the magnitude of cardiovascular (CV) drift and decrease in maximal oxygen uptake (V˙O2max) would be greater at 35°C than at 22°C.The increase in HR and decrease in stroke volume (SV) between 15 and 45 min of cycling at 59.2 ± 1.9% V˙O2max (CV drift) was measured in hot (HEAT, 35°C) and cool (COOL, 22°C) ambient temperatures in 10 endurance-trained men (age = 23 ± 3 yr, V˙O2max = 64.7 ± 8.7 mL·kg−1·min−1). V˙O2max was measured immediately after the 45 min of cycling and again under both ambient temperature conditions on separate days after 15 min of cycling. This design permitted assessment of V˙O2max between the same time points that CV drift occurred. Fluid to replace sweat losses was provided during all trials.CV drift and the associated decrease in V˙O2max was greater (P < 0.05) in HEAT versus COOL. HR increased 11% (P < 0.05), SV decreased 11% (P < 0.05), and V˙O2max fell 15% (P < 0.05) between 15 and 45 min in HEAT, whereas HR and SV changed less (+2% and −2% for HR and SV, respectively, P < 0.05), and there was no significant decrease in V˙O2max (5%, P > 0.05) between 15 and 45 min in COOL.These data demonstrate the magnitude of CV drift during prolonged submaximal exercise, and the accompanying decrease in V˙O2max measured immediately thereafter is greater in a hot than in a cool environment.