L-Arginine is an essential amino acid for infants and growing children. This amino acid is a substrate for at least five enzymes identified in mammals, including arginase, arginine-glycine transaminase, kyotorphine synthase, nitric oxide synthase (NOS) and arginine decarboxylase. L-Arginine exerts antihypertensive and antiproliferative effects on vascular smooth muscles. NOS and arginine decarboxylase appear to be important for the effect of L-arginine on the circulatory system, since each produces nitric oxide (NO), a potent vasodilator, and agmatine, an endogenous noncatecholamine ligand for central alpha-2 adrenoceptors, from L-arginine. Several issues must be clarified before the mechanisms by which L-arginine exerts its effects on the circulatory system can be fully understood.