An in vivo determination of the phenylalanine-4-hydroxylase (E. C. 1.14.16.1) activity is described. Subjects were loaded wit deuterated L-phenylalanine-d5 (200 mg/kg), and the deuterated tyrosine and deuterated phenylalanine in plasma was analyzed using mass fragmentography. Six phenylketonurics (PKU), four hyperphenylalaninemics and two healthy controls were investigated. This method allowed a specific differentiation between PKU's, hyperphenylalaninemics and health controls. The remaining enzyme activity in hyperphenylalaninemics and in PKU patients can be estimated with relatively high accuracy. The hyperphenylalaninemic patients showed 7--17% of the phenylalanine-4-hydroxylase activity found in the two control persons. The PKU patients under diet showed approximately 2--3% of the activity found in the control group. In the PKU patients, loaded while showing high phenylalanine blood concentrations, no remaining activity could be measured. The logarithm of phenylalanine-d5 over tyrosine-d4 in plasma 1 h after loading gives the best differentiation. One single plasma sample of approximately 0.5 ml is sufficient.