The effect of the chronic feeding of methionine or choline on liver tumor promotion by phenobarbital (PhB) or 1,1 bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) was studied in rats receiving an initiating dose of diethylnitrosamine (DEN). Male weanling rats were injected i.p. with DEN (200 mg/kg body wt). Control rats were injected with saline. Five days after the injection, the rats were placed on different diets containing 0.05% PhB or 0.05% DDT with or without added 1.5% DL-methionine or 1.0% choline chloride. Each diet was administered for 72 weeks, when the animals were placed on the unsupplemented chow diet for an additional 30 weeks. Rats treated with DEN and then fed PhB or DDT developed an 85-100% incidence of hepatocellular carcinomas (HCCs). Single injection of DEN alone produced a 60% incidence of HCCs. Dietary feeding of methionine and choline either alone or in combination with PhB or DDT did not have any significant effect on the incidence of HCC's. Liver tumor formation was negligible in uninitiated rats. Lung metastases developed in 42% and 46% of the DEN + PhB- and DEN + DDT-treated groups, respectively. Supplementation of methionine in the diet lowered the incidence of lung metastases to 14% in the DEN + PhB-treated rats and to 19% in DEN + DDT-treated rats. Choline was not effective in inhibiting the development of lung metastases in either case. The injection of DEN alone produced a 54% incidence of lung tumors. PhB and DDT feeding lowered the DEN-induced lung tumor incidence to 23% and 14% respectively. Further, when the data from different diet groups were combined it was found that single injection of DEN also doubled the incidence of leukemia normally seen in F344 rats. The present report constitutes the first evidence that a single injection of DEN induces lung tumors and enhances the incidence of leukemia in rats.