Mitochondrial gene expression has been investigated in synaptic endings from rat cerebral cortex isolated at various stages during the postnatal development and maturation of the animal. The pattern of the mitochondrial translation products labeled in vitro in rat brain synaptosomes revealed some distinctive features when compared with the pattern observed in a rat fibroblast cell line, the most remarkable being the apparent absence of labeling of the ND5 product. This absence contrasted with the presence in synaptosomes of an amount of ND5 mRNA comparable with that found in the rat fibroblast cell line. The rate of mitochondrial protein synthesis per unit amount of mtDNA inbrain synaptosomes showed a characteristic reproducible burst at 10...