The morphological changes induced by a carcinogenic agent have been studied by serial histological examinations on the injection-sites in subcutaneous tissue of mice. As a carcinogen, 4-hydroxy-aminoquinoline-1-oxide was freshly suspended before use in olive oil (0.1 mg/0.1 ml) and injected subcutaneously into the right thigh of mice sixteen times at seven days intervals. Tumors were induced immediately after the sixteenth injection. Based on light and electron microscopic findings, these tumors were considered to be rhabdomyosarcomas. Morphologically, the course of the tumor induction was summarized as follows. The stage of inflammation (1-4 weeks), the stage of necrosis (4-8 weeks), the stage of the appearance of atypical cells (8-15 weeks), and the stage of tumor development (16 weeks and thereafter).