A technique for measuring the gaseous environment during the reconstitution of a large bone defect is described. Extensive testing of this system over the past 10 years has demonstrated its validity in measuring the average extracellular tissue pO-2 and pCO-2. Histology of the tissue surrounding the tonometer was obtained and correlated with the gaseous measurements. New bone formation in the healing segmental bone defects has been shown to take place under hypoxic conditions. The oxygen consumption of the surrounding tissue was determined and correlated with the histology and tissue gas measurements. It was demonstrated that the oxygen consumption was not elevated during the period of new bone formation, reflecting a state of anaerobic metabolism. The hypoxic conditions persist until the vascularity increases to match the cellularity. Thephysiological role of oxygen in osseous repair is presented. The present technique as well as the previously published microelectrode technique have demonstrated bone formation in vivo can take place under hypoxic conditions but the mechanism is not known and requires further investigation.