The development of fibrocartilage in rat lumbar intervertebral discs has been correlated with an immunohistochemical analysis of the changing distribution of extracellular matrix components. Disc anlagen were first recognised by embryonic day 14 as segmental cell condensations. By E16, the notochord formed a series of bulges, each representing a future nucleus pulposus, and the annulus fibrosus had differentiated in the disc anlagen. The inner part of the annulus was composed of cartilage which linked that of adjacent vertebral bodies. The outer part was fibroblastic, with layers of parallel fibroblasts. The long axes of the cells in successive layers lay at an angle of approximately 90 degrees to each other. This criss-cross orientation of cells preceded the oriented deposition of collagen fibres to form the lamellae. Disc anlagen were immunolabelled weakly for types I and III collagen, chondroitin 6-sulphate and dermatan sulphate. Later tissue differentiation was marked by the appearance of type II collagen, chondroitin 4-sulphate and keratan sulphate in the inner annulus. These components also appeared in the outer annulus, but only in adult animals, and indicated metaplastic change in the lamellar fibroblasts. Fibrocartilage in the nucleus pulposus was only seen in old animals, and the origin of the tissue was less clear. However, the fibrocartilage cells appeared to be derived from the cartilage end plate and/or from the inner annulus. We conclude that fibrocartilage in the intervertebral disc is derived from several sources and that the radial distribution patterns of extracellular matrix components in the adult disc are explained by the embryonic origins of its parts.