Metanil yellow, a non-permitted colour for food commodities, is used in the leather, paper and textile industries. In this paper the effect of oral and parenteral administration of Metanil yellow on hepatic and intestinal biochemical parameters was investigated. Oral administration of Metanil yellow (430 mg kg Ψ1 body wt.) for 7 days caused significant depletion of hepatic and intestinal glutathione levels (33-52%) with a concomitant increase in lipid peroxidation (49-121%). Metanil yellow treatment for 7 days also led to a significant increase in cytochrome P-450 (P-450)-dependent aryl hydrocarbon hydroxylase (AHH) activity (99-223%) in the liver and intestine. Cytosolic glutathione-S-transferase (GST) (32-136%) and quinone reductase (QR) (20-92%) activities were also found to be substantially induced in hepatic and intestinal tissues following oral treatment of Metanil yellow. It is interesting to note that oral treatment of Metanil yellow showed a greater response in cytosolic enzymes of hepatic tissue as compared to intestine. Single parenteral administration of Metanil yellow (80 mg kg Ψ1 body wt.) caused significant induction of P-450 and its dependent monooxygenases. Even after 5 days of single parenteral administration of Metanil yellow, hepatic AHH activity showed an elevation of 48% while other monooxygenases were marginally increased. Cytosolic GST and QR showed respective peak inductions of 92% and 60% after 2 and 3 days of parenteral administration of Metanil yellow, which levels off after the 5th day. It can be concluded that Metanil yellow acts as an inducer of a specific form of microsomal P-450 and cytosolic GST and QR, which may involve a cytosolic Ah receptor.