Plasma and cerebrospinal fluid amino acid values were determined in 29 infants 9 to 30 days of age with a confirmed diagnosis of phenylketonuria. Phenylalanine concentrations in plasma and cerebrospinal fluid were markedly elevated; the degree of elevation in the cerebrospinal fluid had a significant relationship to that of the plasma. The only other significant deviations in the plasma were reductions in the threonine and tyrosine values. Cerebrospinal fluid threonine, alanine, and arginine concentrations were reduced, whereas those of serine, isoleucine, and histidine were elevated. This combined deficiency and excess of amino acids in the central nervous system may have a significant effect on protein synthesis at a time in life when this synthesis and turnover is most active.