The purpose of this study was to examine the thermoregulatory sweating control parameters of threshold temperature and sensitivity to determine whether 1) these variables were altered by hypohydration level and exercise intensity and 2) these alterations, if present, were additive and independent. Nine heat-acclimated men completed a matrix of nine trials: three exercise intensities of 25, 45, and 65% maximal O2 uptake and three hydration levels, i.e., euhydration and hypohydration (Hy) at 3 and 5% of body weight. During each trial, subjects attempted 50 min of treadmill exercise in a warm room (30 degrees C dry bulb, 50% relative humidity) while esophageal temperature and upper arm sweating rate were continuously measured. Hypohydration was achieved by exercise and fluid restriction the day preceding the trials. The following new findings were made: 1) threshold temperature increased in graded manner with hypohydration level (approximately 0.06 degree C/% Hy); 2) sensitivity decreased in a graded manner with hypohydration level (approximately 0.06 units/%Hy); 3) threshold temperature was not altered by exercise intensity; and 4) sensitivity increased from low- to moderate- and high-intensity exercise. We conclude that both hypohydration level and exercise intensity produce independent effects on control of thermoregulatory sweating.