Injection of L-p-tyrosine (800 mg/kg, 2 h) increased the mouse striatal para-hydroxyphenylacetic acid (p-HPAA) concentrations. A smaller dose of D,L-m-tyrosine (20 mg/kg, 2 h) produced a larger increase in mouse striatal meta-hydroxyphenylacetic acid (m-HPAA) concentrations. The administration of L-phenylalanine to mice caused a slight increase in the p-HPAA concentration in the corpus striatum after 2 h while a larger dose of L-phenylalanine (800 mg/kg) produced a greater increase. Eight hours following L-phenylalanine injection, p-HPAA concentrations were still elevated. With D-phenylalanine a significant increase was observed at eight hours after drug administration. Two drugs which reduce dopamine synthesis, alpha-methyl-para-tyrosine and apomorphine, decreased m-HPAA striatal concentrations without affecting p-HPAA concentrations. From these results, it is proposed that tyrosine hydroxylase activity determines p-HPAA concentrations by regulating p-tyrosine availability. This enzyme may also synthesize m-tyrosine which is subsequently decarboxylated to form m-tyramine and then oxidatively deaminated to form m-HPAA.