Aristolochic acid (AA) is present in at least 65 different kinds of plants, many of which are used as herbal folk remedies. AA is considered one of the most potent plant carcinogens in humans and animals. It has been associated with the development of urothelial cancers in humans, and kidney and forestomach tumors in rats. In the present study, we used the Big Blue transgenic rat model to evaluate the mutagenicity of AA in kidney of rats and to define the mechanism of action for the tumor induction by AA. Groups of six male Big Blue transgenic rats were gavaged with 0, 0.1, 1.0 and 10.0mgAA/kg body weight 5 times a week for 12 weeks, a treatment protocol that resulted in tumors in kidneys and other tissues. The animals were sacrificed 1 day after the final treatment and the kidneys were isolated for assays to determine the mutant frequencies (MFs) and types of mutations induced by AA in the transgenic cII gene. AA treatment resulted in a strong linear relationship between MF inductions and treatment dose (R(2)=0.998). The cII MFs were 29+/-6x10(-6), 78+/-21x10(-6), 242+/-104x10(-6) and 1319+/-360x10(-6) in the control, low, medium and high dose treatment groups, respectively (p<0.001 for all pair wise comparisons among the four treatment groups). These MFs correlated strongly with tumor incidences induced by the different doses of AA (Mengs et al., 1982). Sequence analysis of the cII mutants revealed that there was a statistically significant difference between the mutational spectra in the AA-treated and control rats (p<0.05). A:T-->T:A transversion was the predominant type of mutation in the AA-treated rats whereas G:C-->A:T transition was the main type of mutations in the control rats. These results suggest that AA induces kidney tumors in rats though a mutagenic mechanism of action.