Analogues of S-adenosylmethionine that were designed as inhibitors of S-adenosylmethionine decarboxylase were tested for their abilities to inhibit the purified enzyme from rat prostate. The most potent inhibitors were 5′-deoxy-5′-[N-methyl-N-[2-(aminooxy)ethyl]amino]adenosine (MAOEA) and 5′-deoxy-5′-[N-methyl-N-(3-hydrazinopropyl)amino]adenosine (MHZPA), which had I50values of 400 nM and 70 nM, respectively, when added directly to the assay medium under standard conditions. These compounds were irreversible inactivators of the enzyme, and more than 95% of the activity was lost within 15 min of exposure to 5 μM MAOEA or 0.5 μM MHZPA. Both inhibitors led to a large reduction in the content of decarboxylated S-adenosylmethionine in LI210 cells and to a substantial decrease in the production of 5′-(methylthio)adenosine by these cells. These results are consistent with their bringing about an inhibition of S-adenosylmethionine decarboxylase activity in the cell which leads to a reduction in the synthesis of spermidine and spermine. Analysis of the polyamine content in LI210 cells exposed to 100 μM MAOEA or 50 μM MHZPA showed that this was the case and that putrescine levels were greatly increased while spermidine and spermine content declined. The combined application of 100 μM MAOEA and 5 mM α-(difluoromethyl)ornithine (an ornithine decarboxylase inhibitor) to L1210 cells completely prevented the synthesis of putrescine, spermidine, and spermine for up to 48 h. The reduction in polyamine content brought about by MHZPA or MAOEA could be partially prevented by the addition of decarboxylated S- adenosylmethionine to the culture medium. These inhibitors also brought about an inhibition of cell growth which could be reversed by the addition of spermidine. These results indicate that inhibitors of S-adenosylmethionine decarboxylase block cell growth by means of their inhibition of the production of spermidine and that putrescine cannot satisfy the requirement for spermidine in the growth of LI210 cells. They also demonstrate that analogues of S-adenosylmethionine are taken up by some mammalian cells and can influence polyamine metabolism. Such compounds have considerable potential as therapeutic agents and for studies of the function of polyamines. © 1988, American Chemical Society. All rights reserved.