While N-nitrosoethylmethylamine (NEMA) is carcinogenic primarily for the liver, its beta-trideuterated derivative, N-nitroso( [2-D3]ethyl)methylamine (NEMA-d3), also produces a high incidence of tumors in the esophagus. To determine whether this shift in organ specificity is associated with an altered pattern of DNA alkylation, [methyl-14C]- and [1-ethyl-14C]-labeled NEMA-d3 were administered to adult male Fischer 344 rats as a single i.p. dose (0.05 mmol/kg; 4 h survival). Levels of methylated and ethylated purines in the DNA of various organs were determined by radio-chromatography on Sephasorb-HP columns. When compared to previous data using undeuterated NEMA, 7-methylguanine levels were found to be reduced by approximately 30% in liver and kidney, but were 160% greater in esophagus. This resulted in a decrease in the 7-methylguanine ratio for liver/esophagus from 109 to 29. O6-Methylguanine was diminished in liver and kidney, but levels in lung and esophagus were too low for quantitative detection. Similarly, deuteration led to an 18% decrease of 7-ethylguanine in hepatic DNA. The observed increase in esophageal DNA methylation correlates with the increased carcinogenicity of NEMA-d3 relative to undeuterated NEMA in that organ. Since pharmacokinetic studies have shown that beta-trideuteration of NEMA does not alter its bioavailability, the data suggest that the observed shift in target organ results from isotopically-induced changes in the balance among competing metabolic pathways in different rat tissues.