A single para osteal injection of an aqueous extract of immature bovine bone doubles the diameter of the rat radius and ulna, within 7 days. Bone is formed by activation and prolifeation of cells in the cambium layer of the periosteum. The fibrous periosteum remains intact and appears to adapt by stretching. New trabeculae are oriented radailly about the diphyses. The organic matrix, mainly new bone collagen is elaborated primarly during the first 72 hours and mineralized, in the last 4 days of study. The response is interpreted as an example of cell activation, rather than osteogenic induction and demonstrates tha rapid and massive osteogensis can be trigegered by an appropriate exogenous stimulus.