The occurrence of spontaneous osteochondroma in rats is extremely rare and only a few cases have been reported. Spontaneous multiple osteochondroma was found in a male SD strain rat, at the age of 58 weeks from the control group in a toxicity study. Histological findings of both a walnut-sized mass involving the humerus and scapula of right forelimb and a soybean-sized mass adjacent to the costochondrial junction of the right posterior rib were similar in appearance. The outer layer of the tumors consisted of hyaline cartilage and the core of the tumors consisted of trabecular bone with abundant fatty bone marrow. The periosteum of the surface was continuous with that of the parent bone, and cortical bone and the medullary cavity of the parent bone communicated with those of the tumors. Because of showing progressive enlargement, multiple osteochondromas in the rat were considered to be neoplastic in nature.