Our objective was to characterise sweat rate responses in a hot environment during rest and subsequent increasing levels of exercise in relation to thermometrically (i.e., rectal, tympanic, mean skin and mean body temperatures) and calorimetrically derived (i.e., change in body heat storage) thermal parameters. Ten healthy males volunteered and entered an environmental chamber set at 42 degrees C. Participants rested seated during their first hour inside the chamber. Thereafter, they exercised to volitional exhaustion on a cycle ergometer at 20 W with step increments of 20 W h(-1). Across time, fluctuations in sweat rate were systematically associated with similar fluctuations in the integral of body heat storage (t = 13.16, P < 0.001), but not rectal (t = 0.98, P > 0.05), tympanic (t = 0.81, P > 0.05), mean skin (t = 0.12, P > 0.05), or mean body (t = 0.93, P > 0.05) temperatures. In addition, 95% limits of agreement and regression analyses showed that the changes in sweat rate demonstrated the highest agreement and strongest associations with changes in the integral of body heat storage. It is concluded that in a hot environment during rest and subsequent increasing levels of exercise sweat rate is associated with the cumulative changes in the rate of body heat storage.