Although ornithine decarboxylase under most conditions is the rate-controlling enzyme of polyamine biosynthesis and thus the most logical target for chemical intervention, the inhibition of the enzyme triggers a series of compensatory reactions all aimed to circumvent the inhibition. These include secondary induction of adenosylmethionine decarboxylase, enhanced accumulation of extracellular polyamines and an overproduction of ornithine decarboxylase resulting from enhanced expression and gene amplification. Thus chemotherapy based on an intervention of polyamine formation has also to be directed to reactions other than the decarboxylation of ornithine. Adenosylmethionine decarboxylase is the second natural target for chemotherapy. Virtually all effective inhibitors of this enzyme are members of the family of bis(guanylhydrazones). Small modifications, such as increased hydrophobicity at the glyoxal portion of the parent compound glyoxal bis(guanylhydrazone), greatly enhance the inhibition of adenosylmethionine decarboxylase and diminish the undesirable inhibition of diamine oxidase. However, although ethylglyoxal and propylglyoxal bis(guanylhydrazone) appear to utilize the putative polyamine carrier for their cellular entry, their cellular accumulation, in contrast to that of glyoxal and methylglyoxal bis(guanylhydrazone), is not stimulated by putrescine and spermidine deprivation produced by inhibitors of ornithine decarboxylase. It is obvious that the cellular accumulation of each of the bis(guanylhydrazones) is determined by their different efflux rates: GBG and MGBG are effectively retained whereas EGBG is rapidly excreted by the tumor cells. GBG and MGBG, but possibly not EGBG, behave as mitochondrial poisons and rapidly produce extensive morphological damage of the mitochondria. The bis(guanylhydrazones) likewise inhibit carnitine-dependent mitochondrial oxidation of long-chain fatty acids, competitively in respect to carnitine. It is possible that this inhibition has something to do with the mitochondrial damage, as carnitine protects tumor cells from the early mitochondrial damage produced by MGBG. Carnitine also protects experimental animals from MGBG-induced acute toxicity and death.