This paper describes the post-natal development of two fibrocartilages in the quadriceps tendon of the rat. The compression-resisting fibrocartilage of the suprapatella was derived from a cell population present in neonates and positioned on the deep surface of the tendon of vastus intermedius. The cells secreted a metachromatic, coarsely fibrous extracellular matrix that was rich in chondroitin sulphate but lacked keratan sulphate or type II collagen. The cells themselves accumulated large quantities of vimentin. The adult form of the suprapatella was attained 8 weeks after birth. The fibrocartilage of the attachment zone of the quadriceps tendon to the patella was formed in a different manner. In animals up to 4 weeks of age, the quadriceps tendon inserted directly into the cartilage model of the patella. When later this was resorbed, and replaced by bone, the cartilage at the attachment zone remained, along with that of the articular surface of the patella. Attachment-zone fibrocartilage was therefore rich in type II collagen, unlike that of the suprapatella. Thus two functionally different fibrocartilages have been shown to have different origins, even when separated by only a short distance within the same tendon. The compositional differences between attachment-zone and compressive region fibrocartilages are also due to their different origins.