Recent research indicates that active transport of sodium in the secretory segment of the sweat gland, followed by the diffusion of water and other solutes into the lumen, forms a sweat precursor fluid. This precursor is modified as it passes through the duct by the re-absorption of sodium producing a more dilute solution. The sweat which is finally expelled at the skin surface however, can vary widely in composition from person to person. Previous work has shown that concentrations can be affected by dietary intake, skin temperature, sweat rate, duration of sweating and methods of sweat collection. Limited information, based on comparable techniques, is available on sweat composition of different ethnic groups. For this study, standard methods for obtaining total body (excluding the head) sweat samples have been used. Results from tests in England, Sharjha, Israel, Nigeria, New Guinea and India have been presented. The ethnic groups studied included both sexes, different age groups and people at various levels of acclimatization. Distinct ethnic differences were found in sweat electrolyte composition. The Indian army groups had the highest sodium/potassium ratios, but most tropical indigenes had much lower values than the British groups. Examination of urine electrolyte excretion rates indicated that many of the differences were due to dietary intake. Females were found to have lower sweat sodium / potassium ratios than males. This difference was found to be independent of sweat rates and urine sodium/potassium ratios. Changes in sweat composition during sweating episodes were examined before and after acclimatization. Increasing sodium concentrations and decreasing sweat rates have been attributed to hydration of the skin. Flatter regression slopes for sweat sodium/potassium ratio on time and on cumulative sweat volume after acclimatization suggest that fatigue of the metabolic process for sweat production may also be involved.