The polyamine biosynthetic enzymes, ornithine decarboxylase and S-adenosylmethionine decarboxylase, are induced in mouse skin after treatment with the tumor-promoting phorbol esters such as 12-O-tetradecanoyl-phorbol-13-acetate. The activity of ornithine decarboxylase is increased at least 200-fold within 4.5 hr after treatment and returns to normal with a half-life of less than 20 minutes. Activity of the second enzyme increases more slowly, reaching a peak about 6-fold over basal level at 12 hours and has yet to return to basal level at 96 hours. A number of tumor-promoting agents also induce activity of both enzymes, but nonpromoting stimulators of cell division such as ethylphenyl propriolate and acetic acid induce only S-adenosylmethionine decarboxylase activity. Ultraviolet light in the carcinogenic range induces both enzymes, but the kinetics of the induction of ornithine decarboxylase activity are much different; maximum induction is not achieved until 24 hr. No evidence for a role for cyclic nucleotides in the induction of the polyamine biosynthetic enzymes by phorbol esters in mouse skin was found. Evidence suggesting a role for microtubules in the induction was obtained; colchicine and certain vinca alkaloids blocked the induction. A number of agents are capable of blocking the induction by phorbol esters of ornithine decarboxylase activity without affecting the induction of S-adenosylmethionine decarboxylase activity. These include 1) retinoic acid, a number of synthetic retinoids, and various naturally occurring retinoids (vitamin A), 2) putrescine and several polyamines, and 3) indomethacin and other inhibitors of prostaglandin synthesis. These agents do not inhibit the induction of S-adenosylmethionine decarboxylase. Insofar as these three classes of blocking agents have been tested, they do inhibit tumor promotion by the phorbol esters. Retinoids have been extensively investigated as inhibitors of tumor formation. Because ornithine decarboxylase is induced by agents that promote tumors as well as by transforming viruses, because the induction is inhibited by agents that inhibit tumor promotion, and because the skin tumors promoted by the phorbol esters as well as many other tumors have a high level of ornithine decarboxylase activity, we have suggested that increased ornithine decarboxylase activity may be an essential part of the mechanism of tumor formation. We suggest that the tumor promoter induces the tumor cell phenotype reversibly, but in those cells that have been initiated, the effect of the promoter is not reversible. It is possible that the mechanism for controlling ornithine decarboxylase activity has been deleted in initiated cells so that the induced level may become permanent and that chronically elevated polyamine levels maintain the growth advantage of initiated cells so that tumors arise.