Abstract:The amino acid content of three tissues was measured in 10‐day‐old rats made hyperphenylalaninemic from age 3 to 10 days by daily injections of phenylalanine plus α‐methylphenylalanine to inhibit phenylalanine hydroxylase (PAH). At 12 h after the last injection, the concentrations of alanine, valine, methionine, isoleucine, and leucine in the cerebral hemispheres were depressed by 25–50%, whereas that of glycine was elevated 2.3‐fold. In the spinal cord, the levels of phosphoserine, methionine, and leucine were decreased by 40–50%, and those of serine and threonine increased by 50%. Tyrosine and phenylalanine concentrations were high in all tissues, 2–3 and 15–30 times normal, respectively; of the amino acids investigated, they were the only ones changed in the liver. Cerebral hyperglycinemia was also produced by chronic treatment with phenylalanine plusp‐chlorophenylalanine to inhibit PAH, but not by acute (12 h) hyperphenylalaninemia. An increase in cerebral phosphoserine phosphatase activity was greater in rats treated with phenylalanine plus PAH inhibitor than with inhibitor alone. The content of brain glycine normally declines with age from birth to 15 days; this decrease was prevented by chronic hyperphenylalaninemia. Attempts to reduce the cerebral glycine content of the hyperphenylalaninemic rats were unsuccessful. However, one of the therapeutic protocols, methionine loading, may be useful because it increased the methionine and decreased the phenylalanine con