In rabbits, repeated passive forcible exercising of immobilized hip, knee, ankle, or elbow joints produced muscle necrosis within a few days. Within two to five weeks, heterotopic cartilage and bone formation occurred in the soft tissues around the joint and in the area of damaged muscle. The changes were morphologically and radiographically similar to those observed in human myositis ossificans. The incidence of these changes was correlated with the duration of immobilization and the frequency of manipulation. Ossification occurred especially in muscle attachments to bone. Denervation, administration of diazepam, or narcosis during manipulation did not prevent the development of heterotopic bone. Treatment by passive manipulation, immobilization, crushing the muscles, or repeated injections of autogenous blood into the muscles did not induce significant calcification. Heterotopic bone formed as a consequence of repeated forcible distention of previously immobilized soft tissues.