Tannin isolated from aqueous extracts of cotton bracts inhibits chloride secretion in canine tracheal epithelial cells by an unknown mechanism(s). Activation of protein kinase C (PKC) by PMA (Phorbol 12-myristate 13-acetale) in airway epithelial cells increases chloride secretion. The effect of tannin on the PKC pathway of chloride secretion was examined using canine tracheal epithelium mounted in Ussing chambers. In tissues pretreated with 1-45 µ/ml tannin, and subsequently stimulated by PMA (10 nM), tannin inhibited PMA stimulation of chloride secretion beginning at a tannin concentration of 9 µ/ml with complete inhibition occurring at a tannin concentration of 45.7 µ/mi. This inhibition was similar to the inhibition produced by H7, a known PKC inhibitor. We conclude that tannin inhibits chloride secretion in airway epithelial cells in part by inhibiting PKC pathways.