Age-dependent morphological alterations of the tibial articular cartilage were studied in 26-month-old Wistar rats of both sexes. Young and adult rats (6 and 10 months of age, respectively) served as a basis for comparison. Light microscopic histology, polarization optical analysis (toluidine-blue staining with or without the addition of 0.2, 0.4 or 0.8 M MgCl2, as well as after phenol reaction) as well as transmission electron microscopy were applied. The main observations are: (1) According to the histological findings, the proportion of the zones changes with age; the subchondral bone layer becomes wider and separated to a certain extent, while the fibres become demasked. (2) Polarization microscopy revealed a disorientation of both the proteoglycane (PG) and collagen molecules; considering the available biochemical data, this phenomenon involves both quantitative and structural changes, as well as differences in the localization of the PG and collagen molecules. (3) Electron microscopy also supports the structural alterations of the collagen (thin and thick, as well as non-striated, fragmented fibres). Cellular alterations are also observed parallel with the changes of the matrix (cell organelles occur less frequently, signs of degeneration and disintegration, rupture of the plasma membrane, as well as accumulation of lipid-bodies can be seen. On the basis of the findings, one can consider the articular cartilage in the old rats as a tissue which is still in equilibrium, however, aged chondrocytes with decreased function become predominant in it. This state can be modified into a pathological one by various mechanical, endocrinological, iatrogenic, post-traumatic, etc., influences through the alterations of the cell-matrix interactions.