We studied liver regeneration after partial hepatectomy and the effects of inhibition of ornithine decarboxylase (ODC) and putrescine administration. The specific ODC inhibitor, alpha-difluoromethylornithine was given as a 3% oral solution (5.4 g/kg X day intake) to hepatectomized rats as well as sham-operated controls. alpha-Difluoromethylornithine had no effect other than inhibition of the low basal levels of ODC in sham-operated rats, but it markedly inhibited increases in ODC by 85% in hepatectomized rats. alpha-Difluoromethylornithine reduced hepatic deoxyribonucleic acid synthesis by 61%, protein synthesis by 46%, and liver weight increased by 83%, showing that alpha-difluoromethylornithine inhibition of ODC inhibits liver regeneration. Putrescine (2 mmol/kg X day) was then given intraperitoneally to hepatectomized rats and controls. Putrescine had no effect in rats not given alpha-difluoromethylornithine. In rats given alpha-difluoromethylornithine, putrescine markedly reversed the inhibitory effect of alpha-difluoromethylornithine on ODC (83%), deoxyribonucleic acid synthesis (94%), protein synthesis (95%), and liver regeneration (85%). These results document that the increases in ODC are important in hepatic regeneration and that polyamine metabolism plays an important role in the increased deoxyribonucleic acid and protein synthesis in this hepatic proliferative response.