Pretest temperature and humidity were correlated with tracheobronchial particle penetration and clearance data from donkeys housed in unheated outdoor facilities and tested after spending 1-2 h in a temperature- and humidity-controlled laboratory. The animals inhaled an inert insoluble radioisotope-labeled monodisperse aerosol for several minutes. Its retention was monitored continuously for 3 h by external gamma detection. Aerosol deposition pattern and bronchial clearance were linearly correlated with pretest outdoor temperature which ranged from -10 to 30 degrees C. The fraction depositing in the unciliated regions of the lung decreased 0.6% per degrees C drop in outdoor temperature. Overall bronchial transport decreased at least 1.5% per degrees C decrease. Multiple linear regression analysis and correction for the positive correlation between temperature and humidity left no significant residual humidity dependence. Acclimatization of the animals in the laboratory for 6 h before testing significantly reduced these effects.