In the canine tracheal epithelium, exposure to cotton bracts extract (CBE) produces significant decreases in transepithelial potential difference (IYms), short-circuit current (Isc), and tissue resistance (R(t)) and a significant increase in mannitol flux, a measure of the paracellular pathway. We investigated the effect of tannin and endotoxin, two major components of cotton bracts extract, on the electrophysiologic properties and shunt pathway of the canine tracheal epithelium mounted in Ussing chambers. Endotoxin (10 µg/ml) had no effect on the canine tracheal epithelium. Tannin in concentrations ranging from 5 to 100 µl/11 ml tissue bath produced significant decreases in IYms and Ise similar to the effects of CBE. Tannin, however, had no effect on Rh and mannitol flux even in concentrations up to 200 µl. The effects of tannin were reversible and specific for the apical cell membrane. These observations suggest that tannin is responsible for most of the decrease in short-circuit observed with CBE but is not responsible foil the loss of integrity of the airway epithelium observed with CBE. In addition, endotoxin alone has no effect on the canine tracheal epithelium.