Ciliated respiratory epithelial cells have to tolerate variations in local pH caused by the respiratory cycle and potential ventilation-perfusion mismatches. We showed previously that peripheral bronchiolar cilia beat at a lower frequency than bronchial cilia, and have now investigated whether they show differences in tolerance to changes in pH. Using the image analysis system applied in the previous study, we compared variations in the ciliary beat frequencies (CBF) of bronchi and bronchioles sampled from human lung resections at various pH in vitro. Application of nonparametric tests (the variance of samples was not similar) indicated that CBF was not significantly modified when pH was varied between 7.5 and 10.5 for bronchi, and between 5.5 and 10.5 for bronchioles. Reversible and significantly lower CBF were observed below pH 7.0 for bronchi and below pH 5.0 for bronchioles. Extreme pH values such as 11.0 or 3.0 were lethal within a few minutes. Thus, respiratory ciliary beating is able to tolerate external pH variations between 3.5 and 10.5 without permanent impairment. In addition we found that alkaline pH values are more favourable than acidic ones and that bronchiolar ciliated cells are more tolerant to acidic pH than bronchial cells.