Rheological studies of the respiratory secretions from patients with cystic fibrosis and from controls are presented. The apparent viscosities of mucous secretions were obtained in five patients with cystic fibrosis and in seven control subjects. The viscosity values for cystic fibrosis mucus after shearing were found to be about 5 times that of controls. The structure of the mucous molecule is discussed, and the various factors affecting its viscosity and flow characteristics are described. The effect of 0.5 molar NaCl solution in dispersing cystic fibrosis mucus is presented. This effect suggests the theory that the high viscosity of mucus in cystic fibrosis is due to a decreased concentration of sodium ions in the structure of the mucous molecule. A link between the high concentration of salt in the sweat and the high viscosity of mucus of patients with cystic fibrosis could thereby be provided.