<p>Alternative method of carcinogenicity assay: In the 1980s and 1990s, carcinogenicity studies in small fish were conducted in various hepatic carcinogens, and were reported to be able to induce carcinogenesis similar to rodents in a short period of time. However, small fish models of carcinogenesis are less sensitive to the carcinogens in organs other than the liver. Thus, they are considered insufficient as an alternative method of carcinogenicity assay. </p><p>Liver carcinogenesis: Medaka is highly sensitive to genotoxic liver carcinogens such as alkylating agents. Medakas were exposed to methylazoxymethanol at 2 ppm for 24 hours and were examined the sequential histological changes. After exposure, the hepatocytes rapidly disappeared, and the liver parenchyma was replaced by regenerative hyperplastic nodules. Tumours were then scattered within them on 60 days post-exposure.</p><p>Swim bladder carcinogenesis: Medaka embryos just before hatching were housed in enclosed glass aquarium within 0 - 6 days post-hatch (dph), in order to deny access to the air-water interface. Uninflated swim bladder was induced in half of the medakas denied access to the air-water interface. All of them had swim bladder adenoma originated from the gas gland. These tumors were observed from 13 dph onwards. The TBARS levels of them were elevated, suggesting that ROS are closely linked to swim bladder carcinogenesis.</p><p>In summary, since medaka carcinogenesis occurs in a very short period compared with rodents, it is suggested that this feature could be useful for biological evaluation involved in carcinogenesis. </p>