Urea cycle disorders (UCD) are a group of 8 rare but devastating inborn errors of metabolism that carry a high mortality and morbidity from the newborn period through adulthood. UCD include deficiencies in any of the six enzymes and two membrane transporters involved in urea biosynthesis: N-acetylglutamate synthase (NAGS);Carbamyl phosphate synthase I (CPSI) deficiency;Ornithine transcarbamylase deficiency (OTCD); Argininosuccinate synthase (AS) deficiency (Citrullinemia);Argininosuccinate lyase (AL) deficiency (Argininosuccinic aciduria);Arginase (ARG) deficiency (Argininemia);Hyperornithinemia, hyperammonemia, homocitrullinuria (HHH) syndrome;and Citrullinemia type II. During the previous grant period we have created the Urea Cycle Disorders Consortium (UCDC) within the Rare Diseases Clinical Research Network (RDCRN) and have launched successfully four research projects aimed at understanding the natural history of UCD and developing new tools for treatment. Currently the UCDC consists of 8 U.S. sites with an interdisciplinary team of over 40 investigators and staff. The consortium works closely with the National Urea Cycle Disorders Foundation, the patient advocacy organization for urea cycle disorders and has collaboration with industry to develop innovative therapies for these disorders. We propose in this application 3 full clinical research projects and a pilot project. In the clinical projects we will: 1) Continue our longitudinal study that investigates the natural history, morbidity, mortality and biomarkers in children and adults with UCD;2) Perform a Phase ll/lll trial of N-carbamylglutamate to assess its efficacy in normalizing ureagenesis in patients with carbamyl phosphate 1 and ornithine transcarbamylase deficiencies;and 3) Assess neural mechanisms of injury in OTCD using structural MRI, functional MRI, and magnetic resonance spectroscopy. In the proposed initial pilot project we will study substrate availability for nitric oxide synthesis and associated pathogenesis in arginase and argininosuccinate lyase deficiencies. In addition to the research studies, we will expand and enhance our website for educational and research resources and continue to provide training and career development opportunities through the UCDC educational programs.