Thermal sweat was examined in 26 hyperthyroid patients before and after the thyrostatic treatment. The mean sweating rate (measured as body weight loss after heat exposure) was a little lower in the state of hyperthyroidism than in euthyroidism; this difference, however, is not significant. The mean sweat sodium concentration also declined but neither was this difference convincing. On the other hand, the potassium concentration in sweat was significantly higher in hyperthyroidism, 9.6 meq/liter in the first sweat sample, than in the post-treatment euthyroidism, 6.7 meq/liter. The relationship between these findings and the exchangeable sodium and potassium pools are emphasized.