Induction of a spontaneously metastasizing carcinoma in rats was attempted. Four-week-old Sprague-Dawley female rats were thymectomized or/and splenectomized and fed 200 mg (20 mg times 10) of 3-methylcholanthrene from 7 weeks of age. In addition to these treatments, the early-appearing tumors were excised in order to select by isoimmunity the late-appearing ones that were less antigenic. The latter were easily transplanted into normal syngeneic female rats with metastasis to remote organs. This metastasizing capacity of the tumor became an inherent character in syngeneic normal rats from generation to generation of transplantation. With one of these tumors (MRMT-1) many cancer cells were histologically detected in circulating blood 3 days after tumor transplantation and arrested in capillary beds of lungs. The spontaneous metastasis to lymph nodes and lungs was macroscopically found within several weeks after tumor transplantation.