In order to determine whether the precursor solution of sweat is abnormal in cystic fibrosis, osmolality and concentrations of sodium and chloride were measured in fluid obtained by micropuncture from the sweat gland coil of the nail fold of patients with this disease. Osmolality was 323+/-4.8 SE (mOsm/kg of water), sodium concentration was 151+/-1.1 SE (mEq/liter), and chloride concentration was 124+/-6.0 SE (mEq/liter). The sweat:plasma ratio for osmolality averaged 1.1+/-0.015 SE. These values are not significantly different from the corresponding ones obtained previously in normal individuals. It is concluded therefore that the disturbance of sweat gland function as far as electrolytes are concerned is restricted to the excretory ducts. In a second series of experiments the stop-flow pressure which is generated by sweat glands during secretion was measured. Values up to 500 mm Hg were found in both patients and normals. According to van't Hoff's law (DeltaP = RTDeltaC) hydrostatic pressure differences of this magnitude can be generated by the osmotic difference of 27 mOsm/kg of water observed between precursor sweat and plasma in the present experiments. With respect to the mechanism of sweat secretion this finding supports the hypothesis that active solute transport creates an osmotic gradient which causes osmotic water flux.