1. A technique for measuring oxygen tension in different zones of the epiphyseal plate, the metaphysis and the diaphysis in vitro and in vivo is described. 2. In the in vitro preparations (costochondral junctions of twenty-one-day-old rats), there is a distinct oxygen gradient corresponding to the morphological zones, beginning with a low tension of 19.5 millimeters of mercury in the zone of small size cartilage cells and increasing progressively to a high tension of 95.2 millimeters of mercury in metaphyseal bone. 3. In sharp contrast, the in vivo preparations (proximal tibial epiphyses of six-week-old rabbits) exhibited a very low-oxygen tension in the zone of hypertrophic cells and in the metaphysis. 4. A steep gradient in oxygen tension was present between metaphyseal bone (19.8 millimeters of mercury) and diaphyseal bone (108.7 millimeters of mercury) in the in vivo preparations. 5. The significance of these findings is discussed, and the physiological role of oxygen in epiphyseal plate growth is examined.