Objective To study the regulation of natural polyamines (putrescine, spermidine and spermine) on ornithine decarboxylase gene transcription in regenerating rat hepatocytes and to assess the roles of polyamines in liver regeneration. Methods Rat regenerating liver was induced by surgical partial hepatectomy (PH). The expressive levels of ODC mRNA in regenerating liver were measured by in situ hybridization analysis. Exogenous polyamines (dissolved in 09% NaCl) were administered subcutaneously to adult male SpragueDawley rats. Results ODC mRNA expression was always lower in high dose (20 mg/kg body weight) putrescinetreated rats compared with that in control rats during the observed regeneration, especially at the 2nd, 4th and 10th hours(P<0.05) after PH; but it was higher in low dose (0.02 mg/kg body weight) putrescinetreated group compared with that in control group only at the 4th and 12th hours(P<0.05) post PH. The mRNA levels in 0.15 mg/kg spermidinetreated group were below the control group in the whole experiment except at the 10th hour postPH; however, 0.03 mg/kg spermidine treatment, first statistically decreasing the mRNA level in the 2 hours after PH, promoted the increase in ODC mRNA at the 4th and 6th hours after PH respectively. The effect of spermine on ODC mRNA expression was similar to that of spermidine.Conclusion Polyamines(especially sper