The apical surface of respiratory epithelial cells is covered by a thin layer of low-viscosity fluid termed airway surface fluid (ASF), about which relatively little is known. We collected samples of ASF from anesthetized rats, which were then analyzed using capillary electrophoresis, a method that enables extremely small quantities of fluid to be analyzed. We found values for Na+ (40.57 +/- 3.08 mM), K+ (1.74 +/- 0.36 mM), and Cl- (45.16 +/- 1.81 mM), indicating that this fluid is hypotonic compared with rat plasma. In contrast, the concentrations of nitrite and nitrate within ASF were higher than reported plasma values. Additionally, intravenous administration of the cholinergic agonist methacholine (MCh) resulted in a dose-dependent increase in the concentration of Na+ and Cl- within the ASF. This increase is approximately 50% in these ions after a dose of 100 ng MCh/g body wt. This animal model, together with this microanalytical technique, may be useful for investigating the in vivo regulation of ASF composition.