Stepronin (SPN) is clinically used as an expectorant, and thenoic acid (TA) is its metabolite. We examined the effects of these drugs on the bioelectric parameters [potential difference (PD), short circuit current (SCC), conductance (G)] of the posterior epithelial membrane of canine trachea and on those of the mucus glycoprotein secretion from feline tracheal isolated glands. PD and SCC were obtained using an Ussing chamber and G was calculated as the ratio SCC/PD. Neither SPN nor TA significantly altered the baseline values of PD and SCC. However, in the mucosal solution, both SPN and TA significantly inhibited PD and SCC evoked by isoproterenol (ISOP), whereas G remained unchanged. Amirolide did not alter the inhibitory action of SPN and TA. Mucus glycoprotein secretion from isolated glands was estimated by measuring trichloride acetic acid-precipitable [3H]-glycoconjugates. SPN and TA significantly reduced mucus glycoprotein secretion. Further, when stimulated by methacholine, these agents significantly inhibited mucus glycoprotein secretion from isolated glands. These findings suggest that SPN inhibits airway secretion in vitro by both decreasing Cl- secretion from epithelial cells and mucus glycoprotein secretion from submucosal glands.