The effects of exposure of articular cartilage to air and the potential for reversibility of the histological and ultrastructural changes that were produced by this exposure were investigated in the knee joint of the rabbit. After a medial parapatellar arthrotomy and lateral dislocation of the patella, the surface of the articular cartilage was exposed to air for one, two, and three hours in forty-five rabbits. Reversibility of the changes was assessed in fifteen rabbits after exposure of the cartilage to air for three hours, closure of the joint, and six weeks of recovery. Histochemical and ultrastructural changes were evaluated, with use of the contralateral non-exposed knee joint as a control. Depletion of glycosaminoglycans in the matrix of articular cartilage, as indicated by a loss of surface staining with toluidine blue, occurred after one hour of exposure to air. Ultrastructural changes occurred in chondrocytes throughout the full thickness of articular cartilage after one hour of exposure. Increases in the time of exposure to air resulted in more pronounced ultrastructural abnormalities in chondrocytes throughout the entire thickness of the articular cartilage, but there was no apparent irreversible cellular injury. Six weeks after arthrotomy, the chondrocytes had fully recovered from the changes that had been noted immediately after exposure to air, and they were devoid of degenerative changes. The cells showed ultrastructural evidence of increased metabolic activity in the nucleus and cytoplasm. In addition, the chondrocytes had partially restored the depleted glycosaminoglycans.