Mammalian spermidine/spermine N1-acetyltransferase (SSAT) was transiently expressed in COS cells. As compared to COS cells transfected with control vector alone, cells transfected with the expression vector containing SSAT cDNA contained lower concentrations of spermidine and spermine. The putrescine content, on the other hand, was markedly increased in the COS cells expressing large amounts of SSAT. These changes in polyamine content were most likely caused by an interconversion of spermine and spermidine into putrescine. The SSAT-induced changes in cellular polyamine content resulted in a compensatory increase in the activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase, i.e. the enzymes catalyzing the rate-limiting steps in polyamine biosynthesis. This is the first demonstration that a primary increase in SSAT activity will induce an interconversion-like change in the polyamine levels and the physiological role of SSAT is most likely to protect cells against too high concentrations of spermidine and spermine.