In adult rabbits, four longitudinal scores (knife cuts) were surgically produced in articular cartilage of the right patella. Post-operatively, they were treated with growth hormone or thyroxine. Histologically degeneration, relatively undiminished orthocromasia, and metachromasia were seen around the scores. In the scores, chondrocytes labeled with 3H-thymidine were seen scattered around the joint. This labeling was more pronounced in the hormone-treated than in the control groups. In the control left knees, almost all joints contained labeled cartilage cells, especially in the hormone-treated groups. In rabbits not operated on, but treated with the same doses of the two hormones, 19 of 20 knee joints contained labeled chondrocytes, whereas no signs of degeneration of the cartilage could be seen. Mitosis of chondrocytes is normally not found in adult articular cartilage and it is evident that the two hormones in some way act as a stimulator of mitotic division.