We have earlier shown that -methylated spermidine and spermine analogues rescue cells from polyamine depletion-induced growth inhibition and maintain pancreatic integrity under severe polyamine deprivation. However, as -methylspermidine can serve as precursor of hypusine, an integral part of functional eukaryotic translation initiation factor 5A required for cell proliferation, and as ,-bis-methylspermine can be converted to methylspermidine, it is not entirely clear whether the restoration of cell growth is actually attributable to hypusine formed from these polyamine analogues. Here, we have used optically active isomers of methylated spermidine and spermine and show that polyamine depletion-induced acute cytostasis in cultured cells could be reversed by all the isomers of the methylpolyamines irrespective of whether they served or not as precursors of hypusine. In transgenic rats with activated polyamine catabolism, all the isomers similarly restored liver regeneration and reduced plasma -amylase activity associated with induced pancreatitis. Under the above experimental conditions, (S,S)-, but not (R,R)-, isomer of bis-methylspermine was converted to methylspermidine apparently through the action of spermine oxidase strongly preferring the (S,S)-isomer. Of the analogues, however, only (S)-methylspermidine sustained cell growth during prolonged (more than one week) inhibition of polyamine biosynthesis. It was also the only isomer efficiently converted to hypusine indicating that deoxyhypusine synthase likewise possesses hidden stereospecificity. Taken together, the results show that growth inhibition in response to polyamine depletion involves two phases: an acute, and a late hypusine-dependent phase.