It has been proposed that the acquisition of efficient energy-transducing mitochondria after birth is mediated by an ATP-dependent mechanism "that causes the rapid maturation of mitochondria without requiring either transcription or translation" (Pollak, J. K., and Sutton, R. (1980) Trends Biochem. Sci. 5, 23-27). Investigation of developmental changes in rat liver mitochondria during the first 6 postnatal h showed that fetal mitochondria had low State 4, State 3, and uncoupled rates of respiration, inefficient coupling between respiration and phosphorylation, and low membrane potentials and proton electrochemical gradients under State 4 conditions. In contrast, hepatic mitochondria from 1-h-old neonates showed increased respiratory control and ADP/O ratios and adult proton electrochemical gradient and membrane potential values. In parallel with these changes, mitochondria became enriched in adenine nucleotides and underwent a 50% reduction in matrix volume. During the first postnatal hour, an increase in mitochondrial succinic dehydrogenase, cytochrome c oxidase, and F1-ATPase activities takes place in the neonatal liver concurrent with a preferential postnatal increase in the in vivo rates of protein synthesis for mitochondrial proteins. In particular, the amount of F1-ATPase increased from 109 +/- 9 to 206 +/- 5 ng/microgram of mitochondrial protein in the first hour of postnatal life. Inhibitors of cytosolic protein synthesis present during the first 2 h of life blocked the postnatal increase in respiratory control and ADP/O ratios, succinic dehydrogenase activity, and F1-ATPase content; but they had no effect on the increase in adenine nucleotide concentrations and mitochondrial volume contraction. This indicates that the acquisition of an efficient coupling between respiration and phosphorylation is dependent on de novo protein synthesis and cannot be brought about by the postnatal increase in adenine nucleotides. The increase of State 4 and uncoupled rates of respiration during the first 2 postnatal h was resistant to protein synthesis inhibitors. We suggest that the postnatal increase in these parameters is due to the reduction of mitochondrial volume occurring during that time, which, in turn, may be triggered by the concurrent enrichment in adenine nucleotides.