Ethanol (20 mM) caused 50-90% inhibition of rates of mixed-function oxidation of p-nitroanisole, 7-ethoxycoumarin and benzo(a)pyrene in perfused rat livers; however, the microsomal metabolism of these substrates was unaltered by low concentrations of ethanol. The metabolism of ethanol was required for this inhibition in the perfused liver. In contrast to ethanol, sorbitol stimulated rates of p-nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats. Both sorbitol and ethanol infusion decreased the hepatic NAD+/NADH ratio; however, the NADP+/NADPH ratio was decreased by sorbitol but increased by ethanol. Stimulation of drug metabolism by sorbitol was abolished by pretreatment of fasted rats with 6-aminonicotinamide, an inhibitor of the pentose phosphate shunt. These data indicated that sorbitol stimulated p-nitroanisole metabolism by providing NADPH via the pentose phosphate shunt. The changes in intracellular concentrations of NADPH produced by ethanol and sorbitol correlated directly with changes in hepatic content of citrate and aspartate. These data suggest that inhibition of the citric acid cycle by ethanol decreases the movement of mitochondrial reducing equivalents into the cytosol via substrate shuttle mechanisms.