Midpoint tenotomy of the mouse Achilles tendon is followed by regeneration with heterotopic ossification at the junction of tendon stump and the regenerated segment. This heterotopic bone formation occurs through a process of endochondral ossification and the cartilage cells appear to arise by a process of fibroblast transformation. Five weeks after tenotomy 60 per cent. of specimens show cartilage and bone formation whilst the remaining 40 per cent. show chondrification only. After 10 weeks all specimens showed bone formation with trabecular bone and haemopoietic marrow. The treatment of tenotomised animals with either disodium ethane‐1,1‐diphosphonate (EHDP) or disodium dichloromethylene diphosphonate (Cl₂ MDP) has no effect on the process of heterotopic ossification which can be detected at the radiological or light microscopical level.