Articular cartilage from the knees of 4 dogs whose ipsilateral paws had been transected 6 weeks earlier (knee_trans), and from their contralateral knees (knee_con), was examined. Kneetrans did not bear weight as a result of the surgical procedure but active motion of the joint, determined with an angular displacement monitor during walking, was comparable to that of knee_con. In comparison to knee_con cartilage, knee_trans samples showed decreases in thickness, Safranin-O staining of the matrix, and uronic acid content (mean, 24.4%), and increase in water content (mean, 5.9%). Incorporation of 35S into proteoglycans was 34–67% less in knee_trans than in knee_con cartilage. Proteoglycan (PG) aggregation in knee_con cartilage was normal, whereas in knee_trans cartilage the bulk of the proteoglycans, and also those that had been newly synthesized (³⁵S-proteoglyeans), did not exist in aggregates. This defect in aggregation was due, at least in part, to an abnormality in the hyaluronatebinding region of the core protein of the proteoglycans, since they did not interact in vitro with hyaluronic acid. These changes are essentially identical to those shown to occur in canine knee cartilage after immobilization of the leg in a cast. Thus, the loading of the joint which occurs from contraction of the muscles that span the knee and stabilize the limb in stance, and not merely joint movement, may be required to maintain the integrity of the articular cartilage.