The aim of this study was to determine the hepatocarcinogenic effect of diethylnitrosamine in rainbow trout (Oncorhynchus mykiss), the most cultivated and consumed fresh water fish. In this study, 120 fish weighing 50-70 g each were used. Randomly selected fish were divided into 3 dosage groups and a control group that contained 30 fish per group. Diethylnitrosaminewas dissolved in 0.1 ml of dimethylsulfoxideat 50 mg/kg, 100 mg/kg and 150 mg/kg, and respectively administered to each of the dosage groups intraperitoneally. On the 15th, 30th and 45th days of the study, 10 fish from each study group were euthanised while under sedation, and necropsies were performed. For histopathological and immunohistochemical examinations, tissues were routinely processed and sections were stained with haematoxylin-eosin and examined by light microscopy. Histology revealed hyperaemia, parenchymal degeneration, mononuclear cell infusion, necrosis, bile ducts hyperplasia, cholangiofibrosis and fatty degeneration of the liver. In immunohistochemical staining, CYP1A was slightly positive in the bile duct epithelium in the C2, A3, B3 and C3 groups, and negative in the other groups. Cytokeratin 18 showed negative staining in all groups. Mild positive staining in the bile duct epithelium, A3, B3 and C3 and negative staining in other groups for glutathione-S-transferase. It was concluded that diethylnitrosamine causes significant damage to liver tissuein fish, even in short-term applications, and may cause neoplastic changes in liver cells over the long term.