1. Measurements of ATP, ADP and AMP concentrations in livers of rats that had been delivered by Caesarian section indicate a rapid shift from a low to a high [ATP]/[AMP] ratio. This change is consistent with the cessation of glycolysis and the initiation of gluconeogenesis at birth. 2. When newborn animals are exposed to a 100% nitrogen atmosphere the hepatic ATP concentration falls and AMP increases. 3. Calculations of the [ATP][AMP]/[ADP]2 ratio give values that are close to the equilibrium constant of adenylate kinase except when the ATP concentration is high. 4. This difference cannot be accounted for by the preferential binding of available Mg2+ to ATP4− rather than ADP3−. It is concluded that the relative proportions of adenine nucleotides at any level of phosphorylation are only partly regulated by adenylate kinase.
