Due to the lack of a complete urea cycle, uricotelic species, such as broilers, are not able to synthesize de novo arginine (Arg), thus depending exclusively on dietary Arg. High levels of dietary lysine (Lys) increase the demand for Arg because of the antagonistic relationship between these amino acids. The Arg-Lys antagonism promotes an expressive increase in the renal Arg activity and consequently induces the degradation of Arg and the decrease in the activity of glycine amidinotransferase, an enzyme that uses Arg in the synthesis of muscle creatin. Arg is considered an important modulator of immunological and physiological processes. The degradation of Arg produces ornithine, a precursor of polyamines that are key to cell division, DNA synthesis, and cell cycle regulation. Arg participates in the synthesis of nitric oxide (NO), a highly reactive free radical in cells and membranes and participates in several cell processes, including in neurotransmission and immune response. Arg is also considered a potent secretagogue of insulin, growth hormone, and IGF-I in the blood stream. Exclusively vegetarian diets may not provide an adequate supply of Arg, which is required for maximum production and for the immune system of current broiler lineages.