Age-related changes are reported in the rat suprapatella: a fibrocartilage that resists compression of the quadriceps tendon against the femur in the flexed knee. The suprapatella was studied by histology, immunohistochemistry and electron microscopy in rats aged 11-14 weeks, and 12, 15, 18 and 24 months. Type II collagen was absent in the matrix of animals 11-14 weeks old, but appeared by 12 months; immunolabelling increased further with age. Chondroitin sulphate was present in all animals, although immunolabelling decreased with age. Keratan sulphate appeared transiently at 12 months. The structure of the suprapatellar cells also changed with age. In some respects the suprapatellar cells of aged rats are similar to those of younger animals; they contain relatively few organelles and their cytoplasm is packed with intermediate filaments which contain vimentin. However, lipid droplets and glycogen are more prominent in older animals, and the nuclei become elaborately infolded and multilobed. Type II collagen was present in rats aged 11-14 weeks in fibrocartilage of the attachment of quadriceps femoris to the patella, but with increasing age it spread proximally, further into the tendon.