Twenty healthy, adult male cats were lightly anesthetized (Nembutal), tracheotomized and were then breathed by a Harvard pump at a fixed frequency and tidal volume. Purified Medical Grade breathing air with or without sulfur dioxide in air or sulfur dioxide in combination with sodium chloride aerosol in air, was delivered to the animals in predetermined exposure sequences and for fixed durations of time. Parameters of response used to judge adaptation of cats to the inhaled challenges of pollutants were pulmonary flow resistance and lung compliance. In addition, pollutant mixtures were delivered to animals via endotracheal catheter and/or face mask to evaluate the possible influence of receptors which may be present in the nasopharyngeal chamber and in the trachea above the tracheal cannula. The major finding was the variability of the responses of the test animals. Certain subjects showed increased pulmonary flow resistance at low SO2 concentration, and were the analogues of the 'reactors' in human populations. Approximately 20 ppm SO2 in air were required to evoke a significant change in pulmonary flow resistance in 'reactors'. The majority of animals showed no response at this concentration of sulfur dioxide in air, either alone or in the presence of NaCl aerosol. When the pollutants were administered via endotracheal catheter and face mask, an increased frequency of significant changes in pulmonary flow resistance in these animals was suggested. All alterations in parameters of response were reversible shortly after exposure ceased. Morphological examination of lung tissue sections after rapid freezing with Freon indicated that measurement of alterations in airway size is not possible in the range of changes of pulmonary flow resistance reported here (< 100%). Methods and data for all experiments are presented in detail.