The role of polyamine metabolism in the regulation of senescence and ethylene evolution was examined in cut carnations. Endogenous concentrations of spermine and spermidine did not change as flowers aged, but putrescine, their immediate biosynthetic precursor, increased dramatically and paralleled a sharp rise in ethylene. When d-arginine, difluoromethylarginine and methylglyoxal bis(guanylhydrazone) were used to inhibit specific steps in polyamine synthesis, ethylene production and the onset of senescence were promoted. In contrast, inhibition of ethylene by aminooxyacetic acid increased the level of spermine, presumably by increasing the availability of aminopropyl groups derived from S-adenosylmethionine (SAM) and required for the synthesis of polyamines from putrescine. These results suggest that the ethylene and polyamine biosynthetic pathways compete for SAM during senescence of carnation flowers.