The ionic composition of the airway surface liquid (ASL) is of importance in cystic fibrosis and exercise-induced asthma. However, literature data on the composition of the ASL vary markedly. The aim of the study was to determine the composition of the ASL, using two different methods involving minimal manipulation. In one method, the composition of the ASL was measured by X-ray microanalysis of frozen-hydrated samples. In the second method, small dextran beads were equilibrated with the ASL in a moisture chamber, isolated, dried, and analyzed. Plasma or serum from the same pigs was also analyzed. Both methods showed that the Na and Cl concentrations in the ASL are close to the concentrations of these ions in plasma. X-ray microanalysis of frozen-hydrated ASL showed significantly higher K, P, and S because here the upper layer (containing cell debris and secreted mucus) is sampled, whereas the bead method samples the watery component of the ASL. Ultrastructural analysis of the epithelium at various osmotic values showed evident damage at concentrations of 50 mM or less. These data support the notion that the physiologically important watery component of the pig ASL has an ionic composition close to that of plasma.