Breast cancer is the major cause of cancer death in women worldwide. Environmental risk factors particularly genotoxic chemicals such as polycyclic aromatic hydrocarbons (PAH) are likely to account for a much higher mortality. Xenobiotic metabolising enzymes in breast tissue are potentially important determinants in both the susceptibility to the mutagenic effects of chemical carcinogens and in the response of breast tumors to chemotherapy. The well known carcinogen 7,12-dimethylbenz(a)anthrazene of PAH family was given (25mg/ml) orally by gastric intubation to induce mammary carcinoma in Sprague-Dawley rats. Increased level of cytochromes (P(450), B(5)), EROD, PROD activities, Phase I biotransformation enzymes (NADPH-cytochrome (P(450)) reductase, NADPH-cytochrome (b(5)) reductase, epoxide hydrolase) and expression of CYP1A1, CYP1A2 and CYP1B1 in liver and breast tissue microsome were documented in DMBA treated group. Phase II enzyme activities (glutathione-S-transferase, gluthatione peroxidase, gluatathione reductase, UDP-glucuronyl transferease) were decreased markedly in cancerous rats. The nut extract of Semecarpus anacardium was administered orally (200mg/kg body wt/day) to the mammary carcinoma rats for 14 days. Drug treatment restored back the altered Phase I and II biotransformation enzymes thus achieving complete detoxification of the carcinogen. These findings suggest that S. anacardium can effectively modulate the catabolism of xenobiotics in rats.