—High circulating levels of phenylalanine caused depletions of threonine, valine, methionine, isoleucine, leucine, histidine, tryptophan, and tyrosine in immature and adult rat brains. The branched-chain amino acids were most affected. Their reductions ranged between 38–64 per cent of control values when phenylalanine was administered either parenterally or in the diet. The pattern of cerebral amino acid depletions found in phenylalanine-injected infant rats was similar to that of the adults. Phenylalanine loading caused depletions in serum amino acid levels in adult rats, but in infant rats the serum levels were either unchanged or somewhat elevated.

Tyrosine, when administered to adult rats either parenterally or via the diet, caused cerebral depletions in essential amino acids, but the depletions were not as striking as with phenylalanine.

In both the infant and adult rat, brain-blood ratios of most of the essential amino acids were significantly reduced by phenylalanine loading.