This study was designed to assess the relative importance of Cl⁻ and HCO₃⁻ secretion to mucociliary transport rate (MCT) in ex vivo porcine tracheas. MCT was measured in one group of tissues that was exposed to adventitial HCO₃⁻-free solution while a parallel group was exposed to adventitial HCO₃⁻-replete solution. After measurement of baseline MCT rates, acetylcholine (ACh) was added to stimulate submucosal gland mucous liquid secretion, and MCT rates were again measured. Before ACh addition, the mean MCT was higher in the HCO₃⁻-free group (4.2 ± 0.9 mm/min) than in the HCO₃⁻-replete group (2.3 ± 0.3 mm/min), but this difference was not statistically significant. ACh addition significantly increased MCT in both groups, but ACh-stimulated MCT was significantly lower in the HCO₃⁻-free group (11.0 ± 1.5 mm/min) than in the HCO₃⁻-replete group (17.0 ± 2.0 mm/min). A second series of experiments examined the effect on MCT of blocking Cl⁻ secretion with 100 μM bumetanide. Before adding ACh, MCT in the bumetanide-treated group (1.0 ± 0.2 mm/min) was significantly lower than in the control group (3.8 ± 1.1 mm/min). ACh addition significantly increased MCT in both groups, but there was no significant difference between the bumetanide-treated group (21.4 ± 1.7 mm/min) and control group (19.5 ± 3.4 mm/min). These results indicate that ACh-stimulated MCT has greater dependence on HCO₃⁻ secretion, whereas the basal MCT rate has greater dependence on Cl⁻ secretion.