SummaryWe determined the activities of tyrosine aminotransferase (TAT, EC 2.6.1.5), p-hydroxyphenylpyruvate oxidase (p-HPPA oxidase, EC 1.14.2.2) and fumarylacetoacetate fumarylhydrolase (FAH, EC 3.7.12) in cytosol of the liver and kidney tissues obtained at autopsy from a case of hereditary tyrosinemia type I. Values were compared with those from a control group of autopsied tissues from three adults and six children, who had died of other causes. In tyrosinemia, these three hepatic enzyme activities were all decreased: TAT showed approximately 35%, p-HPPA oxidase 11%, and FAH 60% of the corresponding control values. On the other hand, kidney enzymes in tyrosinemia revealed that FAH was most significantly decreased to approximately 14% of the control activity. Kmvalues for substrate— determined for p-HPPA oxidase and FAH—were not different between the patient and controls, suggesting no altered properties of these enzymes. We conclude that in the present case of hereditary tyrosinemia type I, the activities of p-HPPA oxidase in liver and FAH in kidney were most strikingly affected. This fact may in part explain the deteriorated metabolism of tyrosine observed in this patient.