Physiological effects of long term inhalation of diesel emissions were investigated in cat lungs. During the first 61 weeks of exposure, the exhaust was diluted to produce a particulate concentration of 6 milligrams per cubic meter (mg/m3). For the balance of exposure, it was adjusted to produce a concentration of 12mg/m3. The animals were exposed 8 hours per day, 7 days per week, for 2 years. Pulmonary mechanical properties were measured. Following the first 61 weeks of exposure, no significant patterns of response were found in mechanical properties, diffusing capacity, uniformity of distribution, or ventilatory performance. A clearly defined response existed at the end of 124 weeks of exposure. The reduction in inspiratory capacity, vital capacity, and total lung capacity with normal values for dynamic ventilatory function indicated a lesion that restricted breathing but did not cause airway obstruction. This restrictive disease was compatible with a diagnosis of pulmonary fibrosis of the interstitial or intraalveolar type. Additional support for the diagnosis was the finding of impaired diffusing capacity. The authors conclude that adverse pulmonary responses occur with high concentrations of diesel emission.