Arterial blood gases, residual lung volume (RV), deflation pressure volume (PV) curves, pulmonary pathology and body weight changes were studied in rats exposed up to 14 days to either 0.75 or 1.0 ppm ozone. Arterial PO2 and body weights decreased progressively with length of exposure while PaCO2 and RV increased. The slope of the PV curve decreased in all groups exposed to ozone. Pathological changes in the lung increased in severity with concentration and length of exposure. The present findings have shown that arterial blood gas measurements represent a sensitive index of altered lung function in rats, a species very sensitive to ozone exposure.