Calvaria from six-day-old infant mice were grown on a grid culture in a chemically defined medium under varying oxygen tensions. Quantitative isotope studies demonstrated a linear association between bone resorption and oxygen tension in the physiological range. This result was supported by histological, histochemical and vital staining experiments. The clinical finding of osteoporosis in areas of hyperaemia could therefore be attributed to a rise in oxygen tension causing increased bone resorption.