Electrolyte concentrations of total body sweat were determined in nine patients with cystic fibrosis of the pancreas and in six controls. The sodium concentration in the patients averaged 79 mEq/l; the controls averaged 18 mEq/l. There was no difference between the two groups in rate of sweating, skin and rectal temperature, ratio of urea in sweat to urea in plasma and lactic acid concentration of sweat. A possible mechanism for the altered composition of sweat is suggested in terms of the present knowledge of sweat physiology.