A new concept, continuous passive motion of a synovial joint in vivo, was investigated to determine its biological effect on the healing of full-thickness articular cartilage defects that penetrate the subchondral bone of knee joints of adolescent and adult rabbits. The effect of continuous passive motion was compared with the effects of immobilization and of intermittent active motion. This investigation included assessment of 480 defects in the knees of 120 adolescent rabbits and assessment of 108 defects in the knees of twenty-seven adult rabbits. The continuous passive motion was well tolerated by these animals, whose general well-being was undisturbed. The healing of the defects at weekly intervals up to four weeks was assessed by gross examination and by an analysis of two indices of healing determined by light microscopy: (1) the nature of the reparative tissue, and (2) the degree of metachromasia of the matrix as demonstrated by toluidine-blue staining. At three weeks this assessment revealed that in the adolescent rabbits, healing of the defects by hyaline articular cartilage was present in 8 per cent of forty defects in ten animals whose knees were immobilized, in 9 per cent of forty defects in ten animals whose knees were permitted intermittent active motion, and in 52 per cent of forty defects in ten animals whose knees were managed immediately after operation by continuous passive motion. At three weeks, in the adult animals, healing of the defects by hyaline articular cartilage was present in 3 per cent of thirty-six defects in nine animals whose knees were immobilized, in 5 per cent of thirty-six defects in nine animals whose knees were permitted intermittent active motion, and in 44 per cent of thirty-six defects in nine animals whose knees were managed immediately after operation by continuous passive motion. Thus, the metaplasia of the healing tissue within the defects from undifferentiated mesenchymal tissue to hyaline articular cartilage was not only much more rapid but also much more complete with continuous passive motion than with either immobilization or intermittent active motion.