Metabolic and enzymatic studies in a patient with hereditary tyrosinemia demonstrated for the first time a deficiency of erythrocyte and hepatic glutathione. Markedly decreased hepatic fumarylacetoacetate hydrolase activity was demonstrated in this patient. The activities of hepatic enzymes not involved in tyrosine metabolism were also determined. Assay of mixed function oxidase activity demonstrated low levels of aryl hydrocarbon hydroxylase and 7-ethoxycoumarin deethylase, suggesting decreased hepatic detoxification capacity. 5-Aminolevulinic acid dehydratase activity was undetectable. Succinylacetone (4,6- dioxoheptanoic acid), an abnormal metabolic product secondary to fumarylacetoacetate hydrolase deficiency was found in serum and urine. Succinylacetone was demonstrated to inhibit 5-aminolevulinic acid dehydratase in vitro, as did the urine, plasma, and red cell Iysates of the patient.