Technical Abstract: Arginine, glutamine, glutamate, proline, aspartate, asparagine, ornithine, and citrulline are interconvertible via complex interorgan metabolism in most mammals (including the pig). The major sites for their metabolism are the small intestine, kidneys, and liver, with cortisol being a key regulatory hormone. Because these amino acids (except for ornithine and citrulline) are usually abundant in plant and animal tissue proteins, pig producers have traditionally paid little attention to supplementing the arginine family of amino acids to swine diets. However, results of recent studies indicate that these amino acids serve important regulatory functions in nutrient metabolism and immune response, thereby affecting the efficiency of feed utilization by pigs. Arginine and glutamine are the prototypes with well-defined functions and expanded applications to pork production. Arginine deficiency, induced by a reduction in intestinal synthesis of citrulline, is a major factor limiting maximal growth of milk-fed piglets. Both enzymological and metabolic studies discovered that low availability of "N"-acetylglutamate in enterocyte mitochondria is responsible for limited synthesis of citrulline from both glutamine and proline in 7- to 21-day-old suckling piglets. Thus, either dietary supplementation with arginine or oral administration of "N"-carbamylglutamate (a metabolically stable analogue of "N"-acetylglutamate) increased muscle protein synthesis and body-weight gain in milk-fed piglets. Moreover, dietary supplementation with glutamine to early-weaned piglets prevented intestinal atrophy and improved growth performance. Remarkably, supplementing arginine to the diet of pregnant gilts between days 30 and 114 of gestation markedly increased the number of live-born piglets and litter birth-weight. Large-scale availability of feed-grade arginine and glutamine holds great promise for improving animal health and nutrient utilization in pig production worldwide.