The metabolic activity of the articular cartilage of the distal end of the femur of adult rabbits was studied by means of isotopic disappearance rates and autoradiography using 35SO4— and glycine-3H. From the data obtained, it is possible to state:
1. The isotopes are probably incorporated into the protein polysaccharide, and their disappearance rates define a metabolically homogeneous fast fraction, having a half-life of about eight days and accounting for over one fourth of the polysaccharide present in the tissue.
2. The rapid degradation is uniformly distributed throughout the articular cartilage, and there is no evidence of focal or polarized loss on autoradiographs. This would tend to invalidate the shedding theory.
3. The rapidity of turnover confirms the presence of the previously described enzymatic internal remodeling system which operates at a rate far in excess of that necessary for compensation of attrition.
4. Cartilage (or at least a large component of the cartilage) is not inert but demonstrates an extremely active anabolic and catabolic system.