The potential carcinogenic effect of inhaled automobile exhaust emissions was examined in rodents. Both rats and hamsters were exposed to the emissions from (1) a gasoline engine, (2) a gasoline engine fitted with a three-way catalytic converter, (3) a diesel engine and (4) a diesel engine with particle filtration. Exposures were for 16 hours per day, 5 days per week, for 2 years. All hamsters were sacrificed at the end of the 2-year exposure period, whereas the rats surviving after 2 years of exposure were maintained for a further 6-month observation period without additional exposure to emissions. Some of the hamsters in each treatment group were pretreated with diethylnitrosamine to induce respiratory tract tumours. No statistically significant changes were seen in the incidence of respiratory tract tumours in emission-exposed hamsters compared to controls. This lack of a treatment-related effect was seen in both the nitrosamine pretreated and the non-pretreated hamsters. There was no increase in the incidence of lung tumours in rats exposed to filtered diesel exhaust or to the exhaust from the gasoline or gasoline-catalyst engines. There was a statistically significant increase in the incidence of lung tumours in rats exposed to diesel engine emissions compared to controls. A clear dose response was evident in both males and females, although the incidence of lung tumours was markedly higher in females (96% in rats surviving beyond 2 years) than in males (44% in rats surviving beyond 2 years). An increased incidence of lung tumours was observed only in rats exposed to mean concentrations of diesel soot particles of either 2200 or 6600 micrograms/m3.(ABSTRACT TRUNCATED AT 250 WORDS)