Polyamines have been suggested to play an important role in stress protection. However, attempts to determine the function of polyamines have been complicated by the fact that, dependent on the conditions, polyamine contents increase or decrease during stress. To determine the importance of polyamine formation during salt stress, we analysed polyamine contents and salt tolerance in two Arabidopsis thaliana mutants, spe1-1 and spe2-1 (Watson et al. Plant J 13: 231-239, 1998), with reduced activity of arginine decarboxylase (EC 4.1.1.19), an important enzyme in polyamine synthesis. Polyamines accumulated in wild-type plants (Col-0 and Ler-0) that were pre-treated with 100 mM NaCl before transfer to 125 mM NaCl, but not in plants that were directly transferred to 125 mM NaCl without prior treatment with 100 mM NaCl. This shows that polyamine accumulation depends on acclimation to salinity. The salt treatment that induced polyamine accumulation in wild-type plants did not lead to polyamine accumulation in the spe1-1 and spe2-1 mutants. Decreased fresh weight, chlorophyll content and photosynthetic efficiency indicated that the spe1-1 mutant was more severely affected by salt stress than its wild type, Col-0. In the spe2-1 mutant decreased salt tolerance compared to its wild type, Ler-0, became apparent as bleaching under severe salt stress. The present results demonstrate that decreased polyamine formation due to lower arginine decarboxylase activity leads to reduced salt tolerance.