2-h-old neonatal liver mitochondria, when depleted or" adenine nucleotides, showed an "ohmic" current--voltage relationship and a higher passive proton i~rmeability of the membrane, resembling fetal mitochondrial behaviors for the proton conductanc©. Incubation of fetal mitochondria with ATP, GDP or carboxyatractylosicle promoted a significant reductio~t in the passive proton permeability of the membrane and the appearance of the characteristic biphasic behavior for the proton conductance. It is concluded that the postnatal increase in intramitochondrial adenine nucleotide corto~'ntration promotes, by the interaction of the nucleotides with the adenine nucleotide translocase, the reduction in the passive proton permeability of the mitochondrial membrane, allowing efficient energy conservation in the neonatal liver.