The effect of N-nitrosodiethylamine (NDEA) on the aquarium fish, Lebistes reticulatus (guppy) has been investigated. The aquarium water contained 10 to 200 parts per million of NDEA. The LC50/30 for NDEA was 100 ppm. Sub-chronic experiments were carried out using 25 and 50 ppm for 90 days. The liver alterations were investigated morphologically. Initially, diffuse necrosis of hepatic cells, disorientation of hepatic structure and cyst-formation appeared. The degenerative lesions were followed by multiple foci of regenerating hepatic cells. In the later period of the experiment, hyperplastic nodules of hepatocytes, cholangiofibrosis, cystic cholangiomas, hepatocellular and cholangiocellular carcinomas could be observed in the treated animals. The following conclusions have been reached: 1. Aquarium fish can be used as a model for short-term, in vivo carcinogenesis experiments. 2. Lebistes reticulatus is a suitable strain for use as a positive control with NDEA. 3. With this model, the process of carcinogenesis can be followed histologically. 4. The process is irreversible and some of its features may be used as an early indicator in the screening of possible carcinogenic substances.