Diet has been found to significantly affect hexachlorobenzene (HCB) accumulation in 20-week old female Osborne Mendel rats. These animals were all fed mg/kg body weight HCB/day for 6, 12 and 18 days, high carbohydrate (67.7% w/w) or high fat (45.3% w/w) diets which were isocaloric with respect to protein. The effect of two sources of carbohydrate, cornstarch and sucrose, on the accumulation of HCB was studied in separate experiments. Feeding the high fat diet not only resulted in higher carcass fat content but also promoted HCB accumulation in the perirenal fat pad, gastrocnemius muscle and liver. these same tissues from obese animals fed the high fat diet had substantially higher fat levels than those fed high carbohydrate diets. Much less HCB was present in the feces of animals fed the high fat ration so the high fat diet presumably facilitated HCB absorption from the gastrointestinal tract. The differences in HCB accumulation in rats fed diets with different carbohydrate sources were slight, but the sucrose diet promoted greater HCB accumulation than the cornstarch diet.