Relative deficiencies of ornithine or arginine occur in the presence of excessive ammonia, excessive lysine, growth, pregnancy, trauma, or protein deficiency and malnutrition. Ammonia excess may occur in the presence of a normal liver when amino acid mixtures lacking ornithine, arginine, or citrulline are infused; when specific amino acids such as glycine are injected; when ammonium salts, urea, or urease are injected; or when the gastrointestinal tract contains an excess of protein, urea, or NH4+, as occurs after a gastrointestinal hemorrhage. In these states, ornithine is often rate-limiting for urea cycle function. Ornithine is also rate-limiting when ammonia excess occurs in the presence of hepatic failure. In three of the inherited urea cycle disorders, ornithine insufficiency and ammonia excess also occur. These disorders are citrullinemia, argininosuccinic aciduria, and argininemia. In the presence of excessive lysine the availability of arginine is reduced and the formation of ornithine is decreased in the liver; urea synthesis is reduced, but orotic acid synthesis is increased, and orotic aciduria results as carbamyl phosphate is directed toward the pyrimidine pathway. Hereditary lysinuric protein intolerance results in ornithine depletion, hyperammonemia, and orotic acid uria. Optimal growth in several species of animals requires 0.4-1.0% arginine in the diet. Diets deficient in arginine are associated with poor wound healing as well as stunted growth. The measurement of orotic acid excretion has been a convenient indicator of insufficiency of ornithine or arginine during growth or pregnancy in animals and should prove useful in assessing the requirement for arginine after trauma. Normal human pregnancy is associated with low-grade orotic aciduria. Protein deficiency and malnutrition increase the vulnerability of the animal or child to ammonia toxicity. This is presumably due to insufficient ornithine for normal urea cycle responsiveness.