A mutation, resulting in a deficiency of liver GTP-cyclohydrolase activity, has been induced in the laboratory mouse. Mice homozygous for this mutation exhibit hyperphenylalaninemia under the following conditions: 1) early in life and 2) throughout life when exposed to phenylalanine. A phenylalanine loading regimen was used to discriminate between mutant and wild type mice on the basis of the resultant phenylalanine and tyrosine serum levels. Subjecting mice to this regimen reveals several distinguishing characteristics. Mutant mice exhibit approximately 2-fold higher peak phenylalanine levels than wild-type mice. In wild-type mice the hyperphenylalaninemic state is transient and rapidly abates while in mutant mice it is persistent and remains for a prolonged period. Mutant mice exhibit normal serum tyrosine levels after a loading challenge, while wild-type mice experience an increase in tyrosine levels. The loading regimen was also used to gauge the response of mutant hyperphenylalaninemic mice to exposure to chemical compounds required for normal phenylalanine catabolism (i.e. pteridine cofactors of the phenylalanine hydroxylase reaction). Mutant mice exposed to native enzyme cofactor or cofactor precursors exhibit a sharp decline in serum phenylalanine levels relative to their uninjected counterparts coupled with a tyrosine increase. By contrast, mutant mice exposed to nonprecursor compounds that are structurally related to the native cofactor, experience no diminution of serum phenylalanine levels.