Several inhibitors of aminopropyltransferases, developed recently in this laboratory, were tested for their specificity by measuring their effects on six enzyme activities related to polyamine biosynthesis and interconversion. Two of them, trans-4-methylcyclohexylamine (4MCHA) and N-(3-aminopropyl)cyclohexylamine (APCHA), selectively and potently inhibited the activities of spermidine synthase and spermine synthase, respectively. They were subjected to in vivo studies using rats. Oral administration of 4MCHA or APCHA dissolved in drinking water (0.02 and 0.1%) available ad lib. for a period of 10 days or 4 months caused a specific and marked decrease in spermidine or spermine in tissues (such as a 95% decrease) with a compensatory increase of spermine or spermidine, respectively, but without any observable change in the growth of the treated rats. Also, with extreme reduction of spermidine or spermine, when their sum was approximately constant, the activity of S-adenosyl-methionine decarboxylase in these tissues was enhanced significantly with no change in the activity of ornithine decarboxylase. These results suggested a separate role for spermidine or spermine in the in vivo enhancement of S-adenosylmethionine decarboxylase activity.