HSIA et al.1 found that plasma phenylalanine-levels were substantially higher after the ingestion of a standard dose, based on body-weight, of this substance by parents of phenylketonuric patients than the levels found in a comparable (age, sex, etc.) normal control group. The same observation was made by Berry, Sutherland and Guest2. Hsia et al. interpreted their results as indicating that a deficiency of the enzyme responsible for phenylalanine metabolism exists in the heterozygous carriers. However, if the data of Hsia et al.1 and Berry, Sutherland and Guest2 on phenylalanine concentration as a function of time are plotted3, it will be seen that the slope of the disappearance curve is essentially the same in the test group and in the control group. The data of Berry, Sutherland and Guest2 actually suggest that phenylalanine is removed more rapidly by the test group than it is by normal subjects.