The importance to mucociliary transport (MCT) and the condition of the mucus of using a heat and moisture exchanger (HME) was investigated. A high tracheotomy was performed on 10 young pigs. The animals were ventilated using a non-breathing system. In five animals an HME was used; the other five were connected directly to the ventilator circuit. After 6 h the trachea was inspected via a flexible bronchoscope. Mucociliary transport velocity was measured using Tc-99-marked macrospheres and a gamma camera. In the control group the tracheal mucus membrane was desiccated at half the distance from the tube tip to the bifurcation. In the HME group the entire trachea was well moistened, but two cases showed large amounts of abnormally thin and foamy secretion. Mucus quality differed significantly between the two groups. Maximum MCT velocities were 8.3 +/- 4.6 and 4.9 +/- 3.0 mm/min for the HME- and the control groups, respectively. The marker had moved 49 +/- 32 and 35 +/- 21 mm during 24 min. Neither of these differences was statistically significant. In the control group there was no transport over desiccated areas. We conclude: 1) the quality of tracheal secretion differed significantly between the two groups, and 2) a heat and moisture exchanger may retain too much water in the airway.