1. A technique for growing epiphyseal-plate organ cultures in a rigidly controlled environment in various oxygen tensions is presented.
2. The cartilage portion of the epiphyseal plate, as determined by macrophotography and tritiated thymidine uptake, exhibited maximum growth in 21 per cent (160 millimeters of mercury) oxygen.
3. Maximum metaphyseal bone formation, as determined by macrophotography, microradiography, and tetracycline staining, occurred in 5 per cent (thirty-eight millimeters of mercury) oxygen.
4. In higher oxygen tensions, the cartilage portion of the plate showed narrowing, a progressive loss of acid mucopolysaccharide stainability, eventual loss of the zone of hypertrophic cells, and an accumulation of neutral mucopolysaccharide or glycomucoprotein at its base.
5. The most sensitive index to high oxygen tensions was an initial, transitory increased length of the zone of hypertrophic cells.
6. The possible mechanisms of oxygen toxicity on epiphyseal-plate growth are discussed.