Maintenance of the aquarium fish, Brachydanio rerio, in tanks containing 10 to 100 parts per million of water-soluble diethylnitros-amine for 8 weeks resulted in irreversible liver damage and hepatic neoplasms. Initial diffuse necrosis of hepatic cells with the formation of intracytoplasmic hyaline masses and ceroid resulted in disorientation of hepatic structure, atrophy, cyst formation, and adenofibrosis. Although the changes were similar to those in rodents fed hepatic carcinogens, neither cirrhosis nor fatty metamorphosis was a consistent finding. With resolution of the necrosis, multiple foci of regenerating hepatic and biliary cells occurred in the livers of nearly all fish, and 17 of 63 had macroscopic, sometimes invasive, hepatomos or cholangiomas between the 10th and 30th week of the experiment. The foci, remarkably diverse in size, structure, cell type, and function were distinguishable from the neoplasms only by their size. The experiments indicate that investigation of carcinogenesis can be done conveniently on small aquarium fish in the conventional laboratory. The high sensitivity of small fish to a known carcinogen, their poikilothermic physiology, and the ease with which their aquatic environment can be controlled are particular advantages in studies on carcinogenesis.