Chronic exposure to aristolochic acids (AAs) from Aristolochia plants is one of the major causes of nephropathy and cancer of the kidney and forestomach. However, the organotropic activities of AAs remain poorly understood. In this study, using LC-MS/MS coupled with a stable isotope-dilution method, we rigorously quantitated for the first time the organ-specific dosage- and time-dependent formation of DNA-AA adducts in the tumor target and nontarget organs of AA-I-treated rats. The results support the proposal that the DNA adduct level is a major contributor to the observed organotropic activities of AAs.