Ornithine aminotransferase (OAT), a mitochondrial, pyridoxal-5`-phosphate-dependent enzyme, plays a central role in L-ornithine and L-Δ¹- pyrroline-5-carboxylate metabolism in animals. This enzyme is widespread in mammalian cells containing mitochondria, and is particularly abundant in the small intestine, liver and kidney. The hormonal and dietary regulation of OAT expression are cell- and tissue-specific, and occur at transcriptional, translational, and pre-mRNA splicing levels. The cell-specific expression of OAT changes developmentally in the liver, kidney and small intestine. The roles of OAT in pathology, physiology and nutrition are largely unknown, but likely depend on cell types and developmental stages. Deficiency of OAT results in hypoornithinemia, hypocitrullinemia and hypoargininemia in infants and neonates but hyperornithinemia in adults, and causes gyrate atrophy of the choroid and retina in adult humans and mice. Future studies are necessary to elucidate the roles for OAT in regulating the synthesis of polyamines, NO, proline and glutamate in mammalian cells under various developmental, physiological, and pathological conditions.