Acute hepatitis spontaneously develops in the Long-Evans Cinnamon rat at the age of 4 mo, and eventually hepatocellular carcinoma develops after the chronic hepatitis that persists for over a year. Previously, abnormal copper accumulation was found in the livers of Long-Evans Cinnamon rats from birth, and it was reported that short-term administration of D-penicillamine, a copper-chelating agent, prevented acute hepatitis in Long-Evans Cinnamon rats. In this study we investigated whether long-term administration of D-penicillamine could also prevent chronic hepatitis and subsequent hepatocellular carcinoma in Long-Evans Cinnamon rats. During long-term observation, which was continued from 11 to 70 wk after birth, no elevation of serum transaminase levels was observed in the Long-Evans Cinnamon rats treated with D-penicillamine. Moreover, no histological changes characteristic of the chronic hepatitis were observed in D-penicillamine-treated Long-Evans Cinnamon rats, which were killed at 70 wk of age. Furthermore, placental glutathione S-transferase-positive foci, described as a marker for preneoplastic lesions in the liver, were not detected, and thus hepatocarcinogenesis was completely prevented in D-penicillamine-treated Long-Evans Cinnamon rats. We also found that the amount of 8-hydroxy-deoxyguanosine, one of oxidative DNA damage products in the liver, was decreased in the Long-Evans Cinnamon rats treated with D-penicillamine. These findings suggest that a process of the prolonged liver-cell injury and regeneration was essential for spontaneous development of hepatocellular carcinoma in Long-Evans Cinnamon rats with abnormal copper metabolism.