Transient synovitis is a commonly occurring affliction in the hips of young children. The results of an experimental model of this condition produced in the immature rabbit's hip showed that there is a significant response in the articular cartilage. Thus, the cartilage rapidly thickens, becomes more hydrated and reveals an initial decrease in glycosaminoglycan concentration. Three weeks following the induction of synovitis, the concentrations of water, glycosaminoglycan and collagen are similar to the control side. The increase of articular cartilage mass is due, not only to an increased hydration, but also to increased matrix production under the stimulus of the synovitis. A similar response by the immature cartilage might be expected following a transient synovitis in a child's hip.