Experiments have been carried out on the bronchi and intrapulmonary airways of rats in vitro, to determine the physiological effects of air with water content (WC) of: 2.19 mg/1 (5% RH (relative humidity) 37 degrees C), 8.77 mg/1 (20% RH, 37 degrees C), 24.13 mg/1 (50% RH, 37 degrees C) and 30.71 mg/1 (70% RH, 37 degrees C) on ciliary beat frequency; and also, the morphological effect of air at temperatures of 30, 40 and 48 degrees at a constant WC of 28.0 mg/l on the ciliated epithelium. The results showed that (i) ciliary beat frequency decreased with decreasing WC and irreversible ciliostasis was reached sooner, the lower the WC value; (ii) ciliary beat frequency was not significantly affected above 24 mg/l; (iii) 45 min and 90 min exposure at 30 degrees C caused an enormous increase in the depth of the mucus film. The cilia and the epithelial cells did not differ from the controls at 40 degrees C; (iv) in airways exposed for 45 min at 48 degrees C, the cilia appeared to be in total disarray. Some were clustered together in whirling arrangements, while others were detached from their bases. The substructure and the cytoplasm of the cells on the surface and in the deeper layers, appeared to have been destroyed; (v) after 90 min at 48 degrees C, the damage to all structures had progressed. Only isolated tissue or cytoplasmic elements were perceptible.