A comparative study was made of 36 aortic enzyme activities (E.A.) and 6 macromelecular substances at different stages of ontogenesis in 49 male rats. In the foetal aorta (19th day) the E.Z. were moderate or weak and restricted to a few metabolic pathways: glycolysis, diaphorases, esterolysis of some nucleotides and glucosaminoglycan (GAG) metabolism. During the neonatal period (1st-3rd day), the pre-existing E.A. increased; some aerobic and lipolytic activities became histochemically detectable; longitudinal and radial gradients became established. These changes seemed to provide evidence of increased morphogenic activities and metabolic exchanges. During the prepuberty and puberty period (10-20th day-2nd month) all the E.A., as well as metachromasis and pyroniophilia increased, and new E.A. appeared (GluDH, GPDH, 5/Nase, Ac.Pase-Ca++, Mg++, pH 7.2). These changes appeared to be related to cell proliferation and connective tissue increase during the period of fast aortic growth. The increase of some E.A. (Est/ase, Ch. est, ATPase-Ca++, Mg++, 5/Nase, Alk. Pase) suggested a correlation between enzymatic differentiation and hormonal maturation. During adulthood (6-12th month), the E.A. were stable except for 5/Nase, lysosomal and lipolytic activities which increased. Some E.A. were found to be high (diaphorases, glycolytic and esterolytic enzymes), or moderate (aerobic oxidoreductases, lysosome, lipolysis and GAG-linked E.A.), while others were weak or absent (glycogen pathway E.A.). These observations seemed to correlate with synthetic processes and defence mechanisms. Ageing (17th month) was characterized by an increase of metachromatic GAG and acid lipids and by a decrease of pyroninophillia. Lysosome, glycolysis and phospholipogenesis-linked E.A. increased. In some animals (individual reactivity) kreb's cycle and lypolysis-E.A. decreased.