Rat livers were perfused with a red blood cell-containing perfusion medium. The rate of metabolism of hexobarbital was determined in this system as the recirculating perfusion fluid was made hypoxic by exposure to either CO or lowered oxygen tension. It was found that 7.6% of the total oxygen consumed by the liver was utilized to metabolize hexobarbital regardless of the severity of the hypoxia, which suggests that the liver does not shunt O2 to more preferential pathways as O2 availability becomes limited. When oxygen delivery was lowered below 0.3 ml of O2 per min per g of liver by either type of hypoxia, the rate of hexobarbital metabolism decreased. However, based on the rate of oxygen delivery, CO was more potent than hypoxic hypoxia in inhibiting drug metabolism. On the other hand, the two types of hypoxia were equipotent when compared on the basis of alterations in oxygen consumption. These results are consistent with the known effect of CO in decreasing the availability of hemoglobin-bound O2 and further argue against the direct inhibition of drug metabolism by the binding of CO to cytochrome P-450 in the isolated liver. They also suggest that the lack of a greater potency of CO relative to hypoxic hypoxia in vivo might be related to alterations in hepatic blood flow.