Abstract: The antimetastatic effects of aryltriazene methoxypyrimidine (ATMP) were investigated in mice implanted intramuscularly with Lewis lung carcinoma in the calf of the hind leg. Primary tumor was removed surgically by amputation in mice treated with the tested compounds preoperatively and postoperatively. The results showed that amputation alone on the 9th or 11th day after tumor implantation did not prevent pulmonary metastasis. Prophylactically antimetastatic effects on spontanous metastasis were observed with the treatment of ATMP when neoplasm spreading had not taken place. It was indicated, with histological examination, that after ATMP administration, the cancer cells of primary tumor entering blood and lymphatic vessels were prevented. Observation with electron microscope showed some ultrastructural changes including the strengthening of the gap junction, more abundant of collagen fibrils and lack of microfilament ,in tumor tissue of ATMP treated group.The selectively antimetastatic effect of ATMP on LA-795 lung adenocarcinoma was also demonstrated.