The development of hepatocellvlar tumors was investigated with histology and electron microscope in 50 males Splague- Dawley rats administered continuously N-nitrosomorpholine(NNM) in drinking water at low dose(5mg/10m1) and high dose(25mg/100ml).

Groups of control and NNM treated rats were investigated for 30 weeks, 2 week intervals beginning of 12 weeks after administration.

Two types of preneoplastic lesion composed of basophilic or glycogenotic hepatocytes were observed preceding the appearance of hepatocellular carcinoma. Hepatocellular cacinoma is increased in number and size according to NNM dosage and duration. Histological classification were trabecular type which is most common, hepatocellular type, adenocarcinoma, and mixed type.

On conclusion, the enviromental contamination of chemical in drinking water might cause of hepatocellular cacinoma and increasd the incidence with larger dosage and longer duration.