The effects of ozone (10028156) on induction of lung tumors by N-nitrosodiethylamine (55185) (DEN) were studied in hamsters. Male Syrian-golden-hamsters were injected subcutaneously with 0 or 20mg/kg DEN twice weekly for 24 weeks. Hamsters were also exposed to 0 or 0.8 part per million ozone 23 hours/day, 7 days/week for 24 weeks. After 24 weeks the hamsters were maintained for another month without any treatment. Mortality was recorded. Surviving animals were killed after 7 months and necropsied. All hamsters injected with DEN survived for 15 weeks, but mortality increased sharply thereafter and by week 29 all DEN treated hamsters were dead. Only two of 16 hamsters exposed to ozone alone and only two of 16 control hamsters died. The incidence of lung tumors in DEN treated and DEN + ozone exposed hamsters was 42 and 21%, respectively. The difference was not statistically significant. The incidence of tumors of the bronchi, trachea, nasal cavity, and liver in DEN treated animals was 29, 96, 33, and 38%, respectively, and was similar to that in the DEN plus ozone group. No tumors were induced by exposure to ozone alone or were found in the controls. Bronchiolar epithelial hyperplasia, alveolar septal thickening, and alveolar septal infiltration by inflammatory cells were seen in the lungs of ozone exposed hamsters. The authors conclude that simultaneous exposure to ozone and DEN does not enhance lung or nasal tumor induction by DEN. Ozone appears to inhibit lung tumor development, although the effect is not statistically significant. The reason for the apparent inhibitory effect of ozone on DEN induced lung tumorigenesis is not clear.