During development the liver undergoes large changes in its oxygen consumption. The energy requirements of the sodium pump (i.e., ouabain-sensitive respiration) would appear to account for a major part of these changes. In very young rats (birth to 9 days), the sodium pump accounted for ~50% of a high weight-specific liver metabolism (LMR). In rats ≥20 days of age, the sodium pump accounted for a smaller percentage (33%) of a much lower LMR. These changes scaled with body mass. LMR scaled with an allometric slope of -0.258, ouabain-sensitive respiration with a slope of -0.355, and ouabain-insensitive respiration with a slope of -0.194. It is proposed that changes in LMR of rats during development are due primarily to the changing energy requirements of the sodium pump.