The oxygen dependence of hepatic cellular respiration was studied by employing simultaneous organ spectrophotometry of cytochromes and hemoglobin, the latter used as an intrasinusoidal optical oxygen probe. The Km of cytochrome aa3 for oxygen was found to be 6.8 μM in the isolated perfused liver and 0.3 μM in suspensions of isolated hepatocytes. The results indicate that the sinusolid-to-cell pO2 gradient is about 5 torr. Optical determination of the average effective pO2 indicates that the axial sinusoidal O2 profile does not conform to zero-order O2 uptake in the liver. Because of extensive NAD+ reduction, ethanol increases the thermodynamic driving force of oxidative phosphorylation, and it also increased the oxygen consumption in both the perfused liver and the hepatocyte suspension, but had no effect on the grade of steady-state cytochrome aa3 reduction, the cellular energy state [ATP]/[ADP]·[Pi], or the Km of cytochrome aa3 for oxygen. The results indicate that hepatic energy metabolism is oxygen independent at very low O2 concentrations, but that the sinusoidal axial O2 concentration is anomalous, probably due to the spatial arrangement of the metabolizing systems.