Crude bracts extract (CBE) and condensed tannin isolated from cotton bracts produced marked changes in the electrophysiologic properties of the canine tracheal epithelium mounted in Ussing chambers. Except at high concentrations, these changes were qualitatively similar and consisted of decreases in short- circuit current (I(sc)) and transepithelial potential difference (YIms) and an increase in tissue resistance (R(t)). For CBE the changes in YIms and R(t) were dose-dependent with a marked decrease in R(t) at high concentrations. For the condensed tannin, a maximal response was observed at 25 µ1/11 ml tissue bath. Changes in electrophysiclogic properties were not reversible but could be stopped when either CBE or tannin were removed from the mucosal bath. These observations suggest that CBE inhibit active ion transport and alter airway epithelial function. Furthermore, the condensed tannin appears to be responsible for these effects at low concentrations while at high concentrations, another compounds in the CBE appears to alter membrane and/or paracellular permeability. These effects could produce changes in secondary water transport and may contribute to the pathophysiology of byssinosis.