Part A. Patients at least 6 months old with ornithine transcarbamylase deficiency (OTC), i.e.: Hemizygous OTC or homozygous autosomal recessive disorder with evidence of complete enzyme deficiency Hemizygous OTC male with late presentation and presumed evidence for residual enzyme activity OTC heterozygotes (molecular diagnosis) with severely symptomatic to asymptomatic disease Obligate heterozygotes for autosomal recessive disorder (parent or genotyped sibling) Normal adult volunteers and genotyped siblings entered as controls Part B. Metabolically stable heterozygous OTC females aged 18 to under 65 Orotic acid level at least 5 times normal on allopurinol Symptoms ranging from severe to asymptomatic acceptable No prior hospitalization for hyperammonemia Exclusion criteria (Parts A and B): Acute or chronic intercurrent illness Pregnancy Acute hyperammonemia