Biotransformation of benzo[a]pyrene (BaP) in the presence of microsomal fractions derived from liver and epiderm of adult and weanling rats was examined. The aim of this study was to evaluate the effect of age on the capacity of two organs in transformation of BaP. Subcellular fractions were prepared from skin and liver by ultracentrifugation and were used as the source of BaP metabolizing enzymes in a reconstitution assay system. Microsomal fractions are sources of cytochrome P-450 and cytosols are the source of glutathione S-transferase (GST). In a metabolic activation assay system, cytochrome P-450 catalyses the formation of reactive epoxide of BaP which can then interact with exogenous DNA. Adult rat liver microsomes with the highest cytochrome P-450 and maximum capacity for BaP-DNA adducts formation (~204 pmol BaP bound/mg DNA) are considered as positive control in this assay system. The adduct formation in the presence of adult and young rats was approximately 204 and 27 pmol/mg DNA, respectively. Microsomes prepared from skin samples of adult and young rats mediated approximately 49 and 16 pmol BaP binding to DNA respectively. With the addition of cytosol to the microsome-mediated system an in vitro detoxification model has been established. The results obtained by the addition of different cytosolic samples showed that liver cytosol which contains highest GST activity caused about 28% inhibition in BaP binding to DNA. The inhibitory effects of cytosolic fraction from weanling liver, adult skin and weanling skin were 17, 19 and ~9% respectively. These data show that isolated subcellular fractions from young rats are less efficient in the biotansformation of BaP. However, the results obtained in vitro do not reflect the changes in vivo. Further, in vivo experiments should be carried out after BaP administration to animals to confirm the differences in the BaP-DNA adduct formation and BaP-glutathione conjunction in tissues of young and adult animals.