Duneclift, S., U. Wells, and J. Widdicombe. Estimation of thickness of airway surface liquid in ferret trachea in vitro. J. Appl. Physiol. 83(3): 761-767, 1997.The tracheae of ferrets and rabbits were mounted in vitro in organ baths. While the tracheae were liquid filled, the permeability coefficient ( P) was determined, and then while the tracheae were air filled, the percent clearance for ^99mTc-labeled diethylenetriaminepentaacetic acid (DTPA) was determined. The thickness of airway surface liquid (ASL) was estimated by three methods. 1) The initial concentration of ^99mTc-DTPA and the total amount of ^99mTc-DTPA (the sum of that entering the outside medium, that draining from the trachea, and that washed out at the end of 40 min) gave the initial volume of ASL and thus its thickness. Mean values were 45.7 µm for the ferret and 41.9 µm for the rabbit. 2) Estimates of ASL thickness at the end of the 40-min period, based on the final ^99mTc-DTPA concentration and the amount in the washout, were 42.9 µm for ferret and 45.4 µm for rabbit. 3) The ratio of P to percent clearance gave mean ASL thickness values of 49.2 µm for the ferret and 40.3 µm for the rabbit. Thus three separate methods for determining ASL thickness give very similar results, with means in the range 40-49 µm. Administration of methacholine or atropine to ferret tracheae did not significantly change ASL thickness.