Isoflavones are thought to be biologically active components in soy that play a role in the prevention of chronic diseases including cancer. How isoflavones may mediate their beneficial effects has not yet been fully established. Potential mechanisms of cellular action of isoflavones may include their ability to modulate gene expression and the activity levels of enzymes involved in antioxidant defence and the metabolism of xenobiotics including NAD(P)H (Nicotinamide-adenine-dinucleotide-phosphate) quinone oxidoreductase 1 (NQO1) and glutathione S-transferase (GST). Although there is increasing evidence from cell culture studies that genistein, the major isoflavone present in soy, may regulate the expression of genes encoding for phase II and antioxidant enzymes, little is known about its effect in vivo. Feeding rats over 3 weeks with semisynthetic diets enriched with genistein (2 g/kg) significantly increased both the hepatic mRNA and activity levels of NQO1. The total GST activity did not change in response to dietary genistein supplementation, whereas the mRNA levels of individual GST isoenzymes were differentially modulated. The hepatic mRNA level of Gsta2 (class alpha 2) was significantly increased whereas the mRNA levels of Gstm2 (class mu 2) and Gstp1 (class pi 1) were significantly lowered due to genistein supplementation. The protein level of Nrf2 (Nuclear factor E2-related factor 2), a transcription factor involved in the regulation of phase II enzymes, was not altered by dietary genistein. Furthermore, genistein did not affect the hepatic enzyme activity of the antioxidant enzymes catalase (CAT), glutathione peroxidase (GPx) and superoxide dismutase (SOD) or liver lipid peroxidation and glutathione levels. The induction of NQO1 may be one mechanism by which dietary genistein improves the capacity of the liver to detoxify carcinogens.