The microheterogeneous nature of intrasynaptic mitochondria has been demonstrated and is widely accepted. However, evidence is still lacking about the role played by the different intrasynaptic mitochondrial subpopulations. The data obtained support the hypothesis that "heavy" mitochondria could represent old mitochondrial populations: in fact, in addition to the well known impairment of typical mitochondrial functions, they possess the highest levels of hydroperoxides and their fatty acids pattern is completely modified. The qualitative and quantitative fatty acid modifications suffered by these organelles deeply altered their protein/lipid ratio, thus modifying their mode of action. The present work also collects a large body of evidence that a subchronic L-acetylcarnitine treatment in 28 days does not structurally affect both nonsynaptic and intrasynaptic mitochondria of normal rat in a "steady-state" metabolic condition.