The enzymes encoded by the polymorphic genes CYP1A1 and CYP2E1 play an important role in the activation and inactivationofxenobiotics.Theseenzymeshavebeenassociatedwithxenobiotic-induceddiseases,suchascancer, therapeutic failure and adverse effects of drugs. The aim of the present study was to determine the allelic and genotypic frequencies of these polymorphisms in a large, ethnically mixed Brazilian population sample from Rio de Janeiro.PolymorphismsCYP1A1andCYP2E1weredeterminedin870unrelatedindividualsbyPCR-RFLPanalysis inperipheralbloodDNA.Theobservedallelicfrequencieswere0.90for CYP1A1*1A and0.95for CYP2E1*1A ,inthe total sample. The allelic frequency of CYP1A1*2C in “pardos” (0.13) and Brazilian whites (0.11) was higher than in Caucasians(0.05),whichmaybearesultoftheAmerindiangeneticcomponent,thatpresentsthehighestfrequency of this allele observed up to now. The genotype distributions for both polymorphisms were in Hardy-Weinberg equilibrium and were statistically different between males and females, and among ethnic groups.