Arginine is a versatile amino acid with multiple metabolic functions in animal cells, including the synthesis of proteins, nitric oxide (NO), urea, polyamines, proline, glutamate, creatine, and arginase. Key enzymes involved in arginine metabolism include argininosuccinate synthase, arginase isoenzymes, nitric oxide synthase isoenzymes, and arginine decarboxylase. Recent studies have highlighted the complex interactions among these enzymes and their roles in determining the metabolic fates of arginine in health and disease. The physiological roles and relationships between arginine synthesis and catabolism are complex, influenced by compartmentalized expression of enzymes at both organ and subcellular levels, as well as changes in expression during development and in response to diet, hormones, and cytokines. The development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches to elucidate the physiological roles of these enzymes. Arginine synthesis occurs primarily in the small intestine and kidney, with the liver also contributing to urea production. Arginine catabolism involves multiple pathways, including the production of NO, polyamines, proline, and glutamate. The regulation of arginine metabolism is influenced by factors such as dietary intake, whole-body protein turnover, and the activity of various enzymes. The arginase isoenzymes, in particular, have been identified as potential regulators of NO synthesis, polyamine synthesis, and other metabolic pathways. Understanding the complex interactions among these enzymes and their regulatory roles is crucial for comprehending the physiological and pathological implications of arginine metabolism.Arginine is a versatile amino acid with multiple metabolic functions in animal cells, including the synthesis of proteins, nitric oxide (NO), urea, polyamines, proline, glutamate, creatine, and arginase. Key enzymes involved in arginine metabolism include argininosuccinate synthase, arginase isoenzymes, nitric oxide synthase isoenzymes, and arginine decarboxylase. Recent studies have highlighted the complex interactions among these enzymes and their roles in determining the metabolic fates of arginine in health and disease. The physiological roles and relationships between arginine synthesis and catabolism are complex, influenced by compartmentalized expression of enzymes at both organ and subcellular levels, as well as changes in expression during development and in response to diet, hormones, and cytokines. The development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches to elucidate the physiological roles of these enzymes. Arginine synthesis occurs primarily in the small intestine and kidney, with the liver also contributing to urea production. Arginine catabolism involves multiple pathways, including the production of NO, polyamines, proline, and glutamate. The regulation of arginine metabolism is influenced by factors such as dietary intake, whole-body protein turnover, and the activity of various enzymes. The arginase isoenzymes, in particular, have been identified as potential regulators of NO synthesis, polyamine synthesis, and other metabolic pathways. Understanding the complex interactions among these enzymes and their regulatory roles is crucial for comprehending the physiological and pathological implications of arginine metabolism.