Summary. Exposure of mice to 0.2 ppm ozone for 2 hrs induces increments in plasma GPT, in liver ascorbic acid and in the number of binuclear lymphocytes in peripheral blood. The effects are most marked at 2 hrs post exposure.The addition of nitrogen dioxide does not influence the ozone effect on the liver ascorbic acid level. Nitrogen dioxide itself also causes an increase in liver ascorbic acid. These effects are preferentially explained by mobilization of an oxidant reducing system rather than by tissue injury alone.