This study was performed to clarify the effects of superoxide dismutase (SOD) on liver regeneration in rats. After about 70% hepatectomy, changes of lipid peroxides (LPO) in the remaining liver tissue, regeneration rate, mitotic index, serum transaminases and ketone body ratio (KBR) were measured under the treatment with and without SOD. The maximal value of LPO of the control group was 14.87 +/- 2.28 nmol/mg protein on the first postoperative day. In contrast, the value in the SOD-treated group was 12.14 +/- 2.38, which was significantly lower than that of the control group. The regeneration rate in the SOD-treated group had been significantly higher than that of the control group from the 4th postoperative day until 4 weeks after the operation when the experiment ended. The mitotic index of the SOD-treated group was also higher than that of the control group on the 2nd postoperative day. Although there was no significant difference in the values of KBR between two groups, the levels of transaminases in the SOD-treated group were significantly lower than those in the control group. These results indicate that SOD treatment suppresses the lipid peroxidation which cause cellular damage after hepatectomy, and hence the mitosis of the liver cell and the liver regeneration can be enhanced.