The shape of long bones is governed to a certain degree by the extent of the surface erosion in the subepiphyseal region. The erosion shows the characteristics of osteitis fibrosa, and may be considered as similar to the initial stage of this disease. The disease may be caused by hyperparathyroidism, but the parathyroid glands are shown to be ill-adapted for the role of "regulators" of normal bone growth. The erosion can also be increased by theelin, and inhibited by testosterone. The balance between these two antagonistic principles probably regulates the intensity of the bone etching and maintains normal control of bone shapes. The greater depth of the fossae often recognizable in female bones may be due to an excess of estrogens. It is suggested that the location of the areas of erosion is determined by the position of the thin-walled veins of the subepiphyseal region.