Studies were conducted on 25 healthy male volunteers aged 20-25 years drawn randomly from the tropical regions of India. The subjects initially underwent an 8 day heat acclimatization schedule with 2 hours moderate work in a climatic chamber at 45 degrees C DB and 30% RH. These heat acclimatized subjects were then hypohydrated to varying levels of body weight deficits, i.e. 1.3 +/- 0.03, 2.3 +/- 0.04 and 3.3 +/- 0.04%, by a combination of water restriction and moderate exercise inside the hot chamber. After 2 hours rest in a thermoneutral room (25 +/- 1 degree C) the hypohydrated subjects were tested on a bicycle ergometer at a fixed submaximal work rate (40 W, 40 min) in a hot dry condition (45 degrees C DB, 30% RH, 34 degrees C WBGT). Significant increases in exercise heart rate and oral temperature were observed in hypohydrated subjects as compared to euhydration. Sweat rate increased with 1% and 2% hypohydration as compared to euhydration, but a significant decrease was observed with 3% hypohydration. Na+ & K+ concentrations in arm sweat increased with increase in the level of hypohydration. Oxygen consumption increased significantly only when hypohydration was about 2% or more. It appears that the increased physiological strain observed in tropical subjects working in heat with graded hypohydration is not solely due to reduced sweat rates.