Barley leaf discs maintained in dark accumulated a massive amount of putrescine (Put), lost chlorophyll and senescenced rapidly. At the same time RNase activity increased significantly. Exogenous spermidine (Spd) inhibited RNase activity, the loss of chlorophyll and degradation of the proteins from thylakoid membranes. Using SDS-PAGE and immunoblot analysis it was shown that spermidine was effective in the retardation of the loss of LHCPII observed in water-treated detached leaves. Analysis of PSII particles isolated from leaf fragments floated in water in the dark revealed the presence of Put, Spd and Spm. In spermidine treated leaves the level of this polyamine in photosystem II was above 5-fold higher than in control. The experimental findings obtained in this study provide evidence that applied spermidine interacts directly with thylakoid membranes so that they become more stable to degradation during senescence.List of abbreviations." Chl -chlorophyll; LHCIIlight harvesting chlorophyll a/b-protein complex; LHCPII -apoprotein of the light-harvesting chlorophyll a/b-protein complex of photosystem II; PAspolyamines; PCA -perchloric acid; PSII -photosystem II; Put -putrescine; Spd -spermidine; Spmspermine; SDS-PAGE -sodium dodecylsulphate polyacrylamide gel electrophoresis.