Male Syrian Golden hamsters were treated with subcutaneous injections of N-nitrosodiethylamine (DEN), 20 mg/kg, twice a week for 24 weeks. Half the animals were kept in filtered air and the other half was exposed continuously to an atmosphere of 0.8 ppm of ozone. After 6 months, no more DEN injections were given and all animals were kept in air until termination of the experiment at 7 months. It was found that the animals kept in ozone developed half as many peripheral lung tumors as did the animals kept in air; however, the difference was not statistically significant. Tumors of the trachea, bronchi, nasal cavity and liver developed with the same incidence whether the animals were exposed to ozone or not. It was concluded that ozone, an agent known to produce cell proliferation in the respiratory tract, does not enhance the development of tumors in the peripheral lung or in the nasal cavity of hamsters.