Injections of 1,3-diaminopropane, a close structural analogue of putrescine (1,4-diaminobutane), into partially hepatectomized rats powerfully inhibited ornithine decarboxylase (EC 4.1.1.17) activity in the regenerating liver in vivo . The compound did not have any effect on the enzyme activity in vitro (under assay conditions employed) but appeared to exert an inhibitory influence on the synthesis of ornithine decarboxylase itself. Repeated injections of diaminopropane into rats after partial hepatectomy, starting at the time of the operation and continued until 33 h postoperatively, markedly diminished the stimulation of ornithine decarboxylase activity in the regenerating liver remnant, and completely prevented the increases in hepatic spermidine concentration normally occurring in response to partial hepatectomy. Treatment of the rats with diaminopropane did not depress the activity of adenosylmethionine decarboxylase (EC 4.1.1.50) in the regenerating liver. Nor did the compound have any effect, whatsoever, on the activity of spermidine synthase (EC 2.5.1.16) in vitro , thus obiviously proving that the increased accumulation of liver spermidine after partial hepatectomy primarily depends upon a stimulation of ornithine decarboxylase activity and a concomitant accumulation of putrescine. The results also showed that 1,3-diamino-propane could not replace putrescine in the synthesis of higher polyamines in rat liver. The inhibition of ornithine decarboxylase by diaminopropane thus appears to represent “gratuitous” repression of polyamine biosynthesis and might conceivably be used for studies devoted to the elucidation of the physiological functions of natural polyamines.