Arginine and its metabolites exert physiological effects on the vasculature and on the kidney and also provide important influences on the regulation of cell proliferation. We summarize the known information regarding two major metabolites of arginine: (a) nitric oxide (NO) and (b) agmatine, decarboxylated arginine. Both agents appear to interact in producing vasodilation and increases in glomerular filtration rate (GFR) in the kidney. There is evidence for inter-regulation of arginine pathways in the sense that agmatine is capable of inhibiting inducible nitric oxide synthase (iNOS), the inflammatory NOS isoform. Both NO and agmatine influence cell proliferation via effects on polyamine synthesis. In addition, both NO and agmatine exert inhibitory effects on ornithine decarboxylase (ODC) and the putrescine transporter by significantly different mechanisms. Therefore, arginine and arginine metabolites exert both vascular regulatory functions and impact on the regulation of cell proliferation. Significant inter-regulation among arginine pathways occurs within the three metabolic major pathways within the cell: (1) nitric oxide synthase (2) arginase and ornithine decarboxylase, and (3) arginine decarboxylase.