Polyamine contents and the activities of their main biosynthetic enzymes, arginine decarboxylase (ADC) and ornithine decarboxylase(ODC), were investigated in Populus leaf segments during adventitious shoot regeneration with the addition of 1 millimolar polyamine synthesis inhibitors. From the study of polyamine synthesis inhibitors, ODC was found to be the principal route of polyamine biosynthesis in adventitious shoot regeneration of Populus leaf segments. The ADC inhibitor difluoromethyl arginine(DFMA) and ODC inhibitor difluoromethyl ornithine(DFMO) strongly reduced the putrescine content. On the contarary, DCHA, an inhibitor of spermidine synthase, increased it. Spermidine content was decreased with the treatment of each polyamine synthesis inhibitor, but the inhibitory effect of DCHA was stronger than any other polyamine inhibitor. The decreased polyamine level by polyamine synthesis inhibitors was restored with the exogenously applied polyamine. Comparing the polyamine contents with the adventitious shoot regeneration rate, were observed a close correlation between spermidine content and adventitious shoot regeneration of Populus leaf segments.