Mice were exposed to 1 ppm O3, 3hrs/day, for 5 consecutive days. A 2-fold increase in % mortality and a 3 day decrease in survival time were observed in mice infected with influenza virus after the 2nd exposure. These endpoints were not affected in mice infected after the 1st, 3rd, 4th, or 5th exposure. O3 had no effect on virus titers in the lungs of mice infected after either the 2nd or 5th exposure. The data indicate that O3 had no effect on anti-viral defenses since virus titers in the lungs were not affected by O3. However, mortality and morbidity, (lungs wet weights, histopathology and pulmonary function changes) were enhanced by O3 exposure in mice infected after the second exposure suggesting that symptoms due to infection can be enhanced in the absence of enhanced virus replication, possibly due to synergistic effects of O3 and virus in production of lung pathologic changes. The fact that effects due to daily repeated exposures were only observed when infection occurred after the 2nd exposure may be due to adaptation.