With aging, rabbit tendon tissue undergoes a series of morphological and biochemical changes which involve both the cells and the extrace-lular matrix. The extracellular matrix increases in volume, causing a relative decrease of the number of cells per unit of tissue surface. The tenoblasts become longer and more slender, while their cytoplasmic processes increase in number and become thinner and more elongated, forming a dense network. In addition, tendon cells show a marked decrease in the intracytoplasmic organelles responsible for protein synthesis, while their intracellular content of contractile proteins does not change. With aging collagen fibers increase in diameter and vary more in thickness. These morphological changes correspond to biochemical changes that include an increase in collagen, a decrease in mucopolysaccharides, and a decrease in water content. During aging parallel changes occur in the elastic fibers, which decrease in number and show structural alterations. CONCLUSIONS Ultrastructural and biochemical studies of tendon diseases need a normal comparison. Out ultrastructural and biochemical findings in aging tendon may be useful in that regard. The presence of actin and myosin in tendon cells could be related to some aspects of tendon physiology and pathology.