The effect of ozone exposure on prostacyclin (PGI2) synthesis in the rat lung was studied. Male Wistar rats were exposed to 0.2, 0.4, 0.8, 1.2 and 1.8 ppm ozone for 24h. The higher concentration (1.8 ppm) significantly depressed-PGI2 synthesizing activity of lung homogenates. Time-courses (1, 3, 5, 7, 14 and 28 days) of the effect of ozone (0.4 and 0.8 ppm) exposure on the PGI2-synthesizing activity of lung homogenates were studied. The PGI2-synthesizing activity of the lung decreased, reaching a maximum at 5 days and then gradually returning to normal by day 14, and remaining normal at day 28, even though the ozone exposure continued. The formation of lipid peroxides due to ozone exposure may cause the depression of PGI2-synthesizing activity of lung. Induction of anti-oxidative enzymes may relate to the recovery of the PGI2-synthesizing activity.