A detailed study of the biochemistry of each of the lower lumbar intervertebral discs from 3 spines aged 8, 16, and 44 years has shown progressive changes down the spine in a number of biochemical parameters. These were most apparent in the 44-year-old spine. The chemical composition of proteoglycans of the nucleus pulposus and of its constituent proteoglycans differed from those of the corresponding annulus fibrosus of all three spines. The interaction of proteoglycans with collagen, as assessed by extractability, changed markedly with advancing age, while the molecular size of the proteoglycans from both regions decreased and their keratan sulphate content increased. These changes would be expected to affect the mechanical properties of the disc.