Ozone and nitrogen dioxide are highly reactive oxidant gases that are derived from the combustion of fossil fuels and the atmospheric transformation of these combustion products. A major unanswered question is whether or not exposure to oxidant air pollutants contributes to lung cancer. Dr. Witschi and colleagues at the University of California at Davis examined whether exposure to ozone or nitrogen dioxide enhances the development of tumors induced by the chemical carcinogen diethylnitrosamine (DEN), particularly neuroendocrine tumors, in the respiratory tract of hamsters. The investigators exposed hamsters continuously to 0.8 ppm ozone for 16 weeks, or 15 ppm nitrogen dioxide for 16 or 24 weeks, or filtered air. The concentrations of the oxidant gases were much higher than those found in ambient air. However, the objective of the study was to determine whether the pollutants had any effects on lung tumor development under extreme conditions before testing ambient levels. Animals also received injections of DEN or saline twice each week. Hamsters exposed to 65% oxygen and injected with DEN were to serve as positive controls. Animals were killed after 16, 24, or 32 weeks, and various tissues were examined by the pathologists, Drs. Schuller and Breider, for neoplastic lesions (carcinomas, adenomas, papillary polyps) and nonneoplastic lesions (hyperplasia and necrosis).