The effects of a single ethanol administration on liver weight gain, RNA synthesis, DNA synthesis, and polyamine metabolism as parameters of liver regeneration in rat liver after partial hepatectomy were studied. Ethanol, given 1 hr before partial hepatectomy at the dose of 3 g/kg, inhibited RNA synthesis 4 hr after partial hepatectomy and DNA synthesis 24 hr after partial hepatectomy in the liver, but did not affect liver weight gain. Further, we studied the effects of a single ethanol administration on polyamine metabolism in order to elucidate the mechanism of the inhibition of liver regeneration by ethanol. Ethanol inhibited the increase in the putrescine level in the liver 4 and 8 hr after surgery, which was a metabolic product of ornithine decarboxylase. But spermidine and spermidine levels in the liver were not affected by ethanol administration. Hepatic spermidine N1-acetyl transferase activity increased by ethanol administration 6 hr after partial hepatectomy in compensation for the suppression of putrescine. Judging from these results, a single dose of ethanol inhibited liver regeneration polyamine metabolism after partial hepatectomy in rats.