Transverse sections of zygapophyseal joints from 61 human, postmortem, lumbar spines of individuals ranging in age from fetal life to 84 years, were used for a study of age changes in relation to biomechanical function. The articular cartilage and subchondral bone of the anterior, coronally oriented third of the joint show changes that are likely to be related to loading of this part of the joint in flexion. The posterior, sagittally oriented two-thirds of the joint shows different age changes, which may reflect shearing forces, imparted to the articular cartilage through the fibrous capsule, from insertion of some fibers of multifidus into the fibrous capsule. The subchondral bone plate, which thickens into a wedge shape with growth to maturity, probably in response to loading stress in flexion, retains this shape into old age despite the bone loss associated with osteoporosis.