A factor inhibitory to sodium transport exists in the sweat of patients with cystic fibrosis of the pancreas. When the duct system of the rat parotid was perfused with sweat from patients, marked inhibition of sodium reabsorption was observed. Perfusion with sweat from normal subjects caused no change in sodium reabsorption. The factor thus demonstrated may be responsible for the increased sodium concentrations in the sweat of patients with cystic fibrosis.