Arginine decarboxylase (ADC; EC 4.1.1.19) is a key enzyme in one of the two possible ways to synthesize putrescine (Put) in plants. In previous work (Masgrau et al. 1997), we observed an altered phenotype (growth inhibition, leaf chlorosis and necrosis) in tobacco transgenic plants (Nicotiana tabacum L. var. Wisconsin-38) containing the oat ADC cDNA under the control of a tetracycline inducible promoter, the severity of which was correlated with Put content. Now we have analysed the T2 generation of a selected transgenic line (line 52), which in previous generations was characterized by presenting a moderate increase in ADC activity and polyamine levels, but no phenotype alterations. Studying two selected individuals, one with a high expression level of the transgene and the other with a moderate expression level, we demonstrate that only the one with increased polyamine content displays the altered (toxic) phenotype. The possible causes of toxicity have been analysed. The results suggest that either Put or its oxidation products, via diamine oxidase (DAO; EC 1.4.3.6), are the responsible factors for the deleterious effects observed in the transgenic plants.