The Swarm rat chondrosarcoma has been the subject of extensive biochemical studies. However, to our knowledge, there are no previous reports in the literature on transplantation of this tumor into bone. This article describes the natural history of the tumor when implanted into the bone of the rat, and correlates its histological growth pattern with its radiological appearance. Our results showed that the tumor grows slowly in the bone. The rate of intramedullary growth, however, was variable and was not the same in all the animals. Its growth pattern resembles human chondrosarcoma, with extensive invasion of the marrow and cortex. In the first few weeks after implantation, the only radiological changes noted were mild medullary radiopacities. At a later stage, 12–14 weeks postimplantation, as the tumor infiltrated the bone, significant radiological abnormalities were observed in the medullary cavity and cortex. Periosteal reaction was seen after the tumor invaded the cortex with the production of a soft-tissue mass. Distant dissemination was rare; only 1 of 24 rats developed pulmonary metastases. The Swarm rat chondrosarcoma is a well-differentiated malignant tumor that histologically resembles welldifferentiated human chondrosarcoma. Transplanted into bone, it may be useful as an experimental model for comparative studies with human chondrosarcoma.