The participation of a multifunctional enzyme (a single polypeptide with multiple catalytic activities (14)) has been demonstrated in the conversion of agmatine to putrescine in Lathyrus sativus seedlings. This enzyme (putrescine synthase) with inherent activities of agmatine iminohydrolase, putrescine transcarbamylase, ornithine transcarbamylase, and carbamate kinase has been purified to homogeneity and has Mr = 55,000. In the presence of inorganic phosphate, the enzyme catalyzed the stoichiometric conversion of agmatine and ornithine to putrescine and citrulline, respectively. The different activities associated with the enzyme copurified with near constancy in their specific activity. The enzyme catalyzed phosphorolysis and arsenolysis of N-carbamyl putrescine. The multifunctionality of putrescine synthase was also supported by 1) activity staining, 2) intact transfer of the ureido-14C group from labeled NJ-carbamyl putrescine to ornithine to form citrulline, and 3) the affinity of the enzyme toward structurally and functionally related affinity matrices. An agmatine cycle is proposed wherein N-carbamyl putrescine arising from the agmatine iminohydrolase reaction is converted to putrescine and citrulline, with the ureido group of N-carbamyl putrescine being transferred intact to ornithine. Preliminary results indicate that this series of reactions is also present in other plants.