Young adult male cats were exposed 28 days, 20 hours per day, to a 1:14 dilution of diesel exhaust emissions. Following termination of exposure, the following pulmonary function measurements were carried out: lung volumes, maximum expiratory flow rates (MEF), MEF at 50%, 25% and 10% of vital capacity (VC): forced expiratory volume (FEV) after 0.2, 0.3 and 0.4 seconds, dynamic compliance, resistance and helium washout at 25, 50, 75 and 100 breaths per minute. The only significant functional change was a decrease in MEF at 10% of VC (P= .02). The lungs of the exposed cats appeared charcoal grey with frequent focal black spots visible for the pleural surface. Pathologic changes in the exposed cats included a predominantly peribronchiolar localization of black-pigmented macrophages within the alveoli producing a focal pneumonitis or alveolitis. In general, evidence of serious lung damage was not observed following the 28-day exposure period.