The age dependence of the relative rate of biosynthesis of intercellular matrix macromolecules was studied in organ culture and cell culture obtained from aortas of newborn, young and adult rabbits. In organ culture there was a strong decrease with age of the rate of incorporation of [14C]-lysine and [3H]-glucosamine in all macromolecular fractions. Neosynthesis of elastin could be demonstrated by the isolation of labelled desmosine at all ages. In cell cultures derived from newborn and adult aortas, no decrease in total incorporation was noticed. The pattern of synthesis and secretion of glycosaminoglycans and glycoproteins did however change with age. These results suggest the existence of a matrix-dependent and of a matrix-independent regulation of the relative rate of synthesis of matrix macromolecules.