Knee joints of mature rabbits were exposed to room air for periods of time ranging from thirty minutes to one hour in an attempt to mimic the human situation in the operating room. The animals were killed after the joint had been closed and activity in a cage had been allowed for twenty-four hours. When the animal was killed, cartilage was removed and was incubated with radioactive proline for four hours before light microscopy autoradiographs were made. Other samples were prepared for study by electron microscopy. The results in the animals that were killed immediately after the cartilage was exposed to room air and in those that were killed twenty-four hours after closure of the joint were identical. Both the ultrastructural and the autoradiographic findings indicated that the entire thickness of the articular cartilage was necrotic after sixty minutes of drying. Forty-five minutes of drying produced complete necrosis of the cartilage in half of the animals. In the other half, some cells survived, although many areas of the cartilage had complete necrosis, top to bottom. Thirty minutes of drying produced patchy necrosis that extended only to the middle zone of the cartilage. In joints that were exposed to room air for one hour, necrosis of the chondrocytes was completely prevented by irrigating the joint every five minutes with Ringer lactate solution.