The LEC rat spontaneously develops liver cancer after suffering chronic liver injury caused by abnormal copper accumulation in the liver, but the role of copper accumulation in the induction of liver cancer remains obscure. We histochemically and biochemically examined the content of copper and metallothionein (MT), a cytoplasmic copper binding protein, in spontaneously developed preneoplastic and neoplastic liver lesions and compared them with those in the surrounding liver tissues. Histochemically, the majority of the preneoplastic liver lesions (68%) and liver cancers (59%) showed lower copper contents than the surrounding liver tissues and no lesions were shown to accumulate more copper than the surrounding tissues. A marked heterogeneity in copper staining was observed in cancer tissues. In contrast, these lesions showed an equal to higher MT content than their surroundings. Biochemical measurements of copper and MT in cancer tissues supported the histochemical findings. The bromodeoxyuridine (BrdU) labeling index was high in all cancer tissues and some of the preneoplastic liver lesions. Parts of the cancer tissues with negative or weak staining for copper were highly labeled with BrdU. Taking these results together, copper accumulation may exert a growth inhibitory effect on surrounding hepatocytes, whereas the hepatocytes in the liver lesions could proliferate, escaping from the effect of copper toxicity by increasing their MT induction and lowering copper accumulation. Thus, accumulation of copper may act as a promoting factor for the development of liver cancer in LEC rats by creating a selective growth environment.