Collagen metabolism in the focal degenerated cartilage from immature dogs with degenerative joint disease was compared with that in the adjacent 'normal' cartilage of the same joint surface. The deposition of collagen into the cartilage in vitro as measured by accumulation of hydroxy[14C]proline was decreased in the early and in advanced stages of cartilage degeneration. The deposition of collagen into cartilage in vivo as measured by the accumulation of hydroxy[3H]proline (intravenously injected [3H]proline) also was reduced in the degenerated cartilages of a dog with degenerative joint disease. Gel electrophoretic analysis revealed that degenerated cartilage contained less alpha 1 II collagen chains, but increased amounts of larger proteins. Degenerated cartilage contained more water, increased amounts of unidentified, non-collagenous protein, increased collagenolytic enzyme activity and fewer chondrocytes. Decreased deposition of collagen would result in collagen depletion in the foci of degenerated cartilage in joints of dogs with degenerative joint disease.