Extension springs were placed across the elbow joints of mature but not senescent rabbits in a manner which produced compression but allowed the joint to move. Compression was maintained for from one to thirty-two days. Within the range of compression used, the severity of cartilage damage correlated with the duration rather than the magnitude of the compression. As the arthritic lesion developed, the cartilage matrix underwent fibrillation and eventually was eroded away entirely, while a proliferative response was evoked at the joint margin. The chrondrocytes became pyknotic and then, in the end stages, were compressed into linear masses devoid of internal structure.