This study examined the influence that aerobic fitness (VO2 max) had on final heart rate (HR), final rectal temperature (Tre), and total body sweat rate (Msw) when subjects exercised while euhydrated and hypohydrated (-5.0% from baseline body weight). Eight male and six female subjects completed four exercise tests both before and after a 10-d heat acclimation program. The tests were a euhydration and a hypohydration exposure conducted in a comfortable (20 degrees C, 40% rh) and in a hot-dry (49 degrees C, 20% rh) environment. Significant differences were not generally found between the genders for HR, Tre and Msw during the tests. In the comfortable environment, HR, Tre and Msw were not generally significantly correlated (p greater than 0.05) with VO2max. In the hot-dry environment, Tre and VO2max were significantly correlated (r = -0.58) when euhydrated before acclimation. HR was significantly related to VO2max before acclimation when eu- (r = -0.61) and hypohydrated (r = -0.60) as well as after acclimation when eu- (r = -0.57) and hypohydrated (r = -0.67). These data indicate that, when euhydrated in the heat, aerobic fitness provides cardiovascular and thermoregulatory benefits before acclimation, but only cardiovascular benefits after acclimation. However, when hypohydrated in the heat, cardiovascular benefits are present for fit subjects both before and after acclimation, but thermoregulatory benefits are not associated with fitness.