Health effects of inhaled diesel engine exhaust were evaluated in infant guinea pigs following 4 and 8 weeks of exposure. Animals were exposed to 1 part exhaust diluted by 13 parts clean air for 20 hr/day, 7 days/week. Lung function, electrocardiogram, growth rate, and histopathology were assessed following exposure. After 4 weeks, animals exposed to irradiated exhaust, showed a 35% increase in pulmonary air flow resistance, and a small but significant decrease in heart rate. Necropsy, after 8 weeks, showed exhaust exposed animals to have black discoloration of the lungs and increased lung to body weight ratios. Microscopic examination of the lungs from these animals revealed black particulate material in the cytoplasm of alveolar macrophages and in draining lymph nodes.