Long-term inhalation studies were carried out to evaluate the toxic pulmonary effects of diesel engine emissions. Cats were exposed for over 2 years to whole, diluted diesel exhaust at levels expected to produce frank toxic effects. During the first 61 weeks of exposure, the cats received exhaust having a particulate level of 6 mg m-3. This was followed by a doubling of the exposure level from weeks 62 to 124 resulting in particulate levels of 12 mg m-3. No definitive pattern of pulmonary function response was observed following 61 weeks; however, a classic pattern of restrictive lung disease was found at 124 weeks. The significantly reduced lung volumes and diffusing capacity were indicative of a pulmonary interstitial response which was later verified by histopathology.