Mice homozygous for dl have been suggested as models for phenylketonuria. We found: (1) the concentration of phenylalanine in the blood was normal at all ages examined; (2) phenylalanine hydroxylase activity in the liver in vitro equalled that in unaffected littermates; (3) the apparent Km values for phenylalanine and cofactor respectively in dl/dl mice were the same as in their normal littermates; (4) inhibition of the overall reaction by the particulate fraction, excess of substrate, excess of cofactor or phenylpyruvic acid showed no difference between dl/dl mice and their unaffected littermates; (5) phenylalanine injected in vivo had equal, small, effects on phenylalanine hydroxylase activity of the liver measured in vitro in the two groups of mice. An explanation of the findings of other workers, based on the natural history of the disease process, is tentatively put forward.
