Impaired sweating reportedly has been associated with a high incidence of heat pyrexia. An important cause is attributed to sweat gland fatigue following intense prolonged sweating. For further investigation, sweat gland fatigue was induced by heat exposure for 2 hours in a hot room (dry bulb 46c, wet bulb 37c) with 9 healthy subjects. Sweating rate of the right forearm increased progressively for the first 45 minutes with a maximum value of 40.0mg/4. 91 cm2/15 min followed by a slow decrease to 24.9 mg/4.91cm2/15 min at the end of 2 hours. Active sweat pores decreased in number following 2 hours of heat exposure but the differences were not significant statistically. In spite of the higher sweat rate on the back, the number of active sweat pores was smaller as compared to the right forearm. Na+ concentrations of the second hour sweat collections were higher than those of the first hour with difference of 14. 9 mEq/l on the right forearm and 12.3 mEq/1 on the back.