Diet has been implicated as a major determinant of chemical carcinogenesis. Accordingly, rates of benzo[a]pyrene (B[a]P) metabolism were compared in hepatocytes isolated from rats maintained on control, high-fat or food-restricted AIN-76A diets. Rats maintained on the food-restricted diet were given 65% of food consumed by the control group fed ad libitum. The high-fat diet group had free access to a modified AIN-76A diet in which the amount of corn oil was increased 4-fold at the expense of digestible carbohydrates. The triacylglycerol content in hepatocytes varied in direct proportion to dietary fat and calories and was 66 +/- 5, 105 +/- 7 and 192 +/- 16 nmol/mg dry wt in cells isolated from rats fed food-restricted, control and high-fat diets respectively. In contrast, the rate of B[a]P metabolism was highest in hepatocytes from rats maintained on the food-restricted diet and lowest in cells from animals given the high-fat diet (i.e. food-restricted greater than control greater than high-fat). Thus, an inverse correlation existed between the rate of B[a]P metabolism and the content of triacylglycerols in hepatocytes. At a cell density of approximately 2 mg dry wt/ml, rates of B[a]P (40 microM) metabolism were 1324 +/- 186, 1150 +/- 198 and 829 +/- 76 pmol/mg dry wt/h, respectively, in hepatocytes isolated from rats fed food-restricted, control and high-fat diets. When cells were incubated with a lower concentration of B[a]P (10 microM), the rate of B[a]P metabolism was greater than 2-fold higher in hepatocytes from rats fed the food-restricted diet compared to the rate measured in cells from the high-fat group. Glucuronidation of B[a]P metabolites in hepatocytes from rats fed high-fat diet was also approximately 30% lower than rates determined for control and food-restricted groups. These diet-induced alterations in rates of B[a]P metabolism occurred in the absence of changes in specific activity of arylhydrocarbon hydroxylase or UDP-glucuronosyltransferase in liver microsomes. Further, the rate of 7-ethoxycoumarin metabolism, a more hydrophilic substrate, was not affected by diet and B[a]P but not 7-ethoxycoumarin accumulated in hepatic lipid droplets. Thus, diet-induced changes in intracellular triacylglycerol, particularly in lipid droplets, may alter access of B[a]P to binding sites on arylhydrocarbon hydroxylase and thereby modulate B[a]P metabolism in intact hepatocytes.