To investigate the effect of yam on the activity of benzo[a]pyrene hydroxylase(BPH), the key enzyme associated with polycyclic aromatic hydrocarbons(PAHs) metabolism, rats were fed a fiber free diet for 7 days, whereupon they were switched to experimental diets for another 7 days. Diets contained benzo[a] pyrene(BP, 400 mg/kg diet) and 25% or 50% yam powder (freezer dried and hot air dried ). Diets containing pectin and cellulose were compared with diets containing yam. BPH activities were assessed in the liver, lung, kidney, stomach, small intestine and large intestine of rats. BP induced BPH activities in various tissues ; 8 fold in liver, 28 in lung and stomach , and 32 in large intestine. The addition of yam significantly lowered BPH activity in liver, lung and stomach and hot air dried yam was nmor eeffectivie than freeze dried yams. These data suggested that yam containing diet may influence carcinogen metabolism in liver and extrahepatic target tissues by altering activities of BPH and may reduce exposure of these tissues to dietary carcinogens.