Free L-Arginine (Arg) is provided by diet, de novo synthesis and protein turnover. De novo synthesis is sufficiently important to not consider Arg as an essential amino acid. However, in some circumstances – eg. early development, infection and inflammation – de novo synthesis is not enough and require dietary Arg, which is thus classified as a conditionally-essential amino acid. Whatever the source, Arg can be metabolized through different degrading pathways which include i) the nitric oxide synthase (NOS) pathway providing Citrulline and NO, as well as ii) the well-known Arginase pathway leading to the production of Urea and L-Ornithine (Orn).