The decarboxylase inhibitor DL-alpha-monofluoromethyldopa reduces, in a dose dependent manner, the concentration of striatal p-tyramine in the mouse. Homovanillic acid is also significantly reduced. Conversely, this treatment increases the m-tyramine concentration. Administration of m-tyrosine produces large increases in m-tyramine and a slight decrease in p-tyramine; these changes are potentiated in the presence of the decarboxylase inhibitor. Such data along with other recently published results permit the conclusion that m-tyramine arises from phenylalanine via m-tyrosine and that p-tyramine arises by decarboxylation of p-tyrosine. Both these reactions are closely related to the activity of tyrosine hydroxylase and the availability of appropriate substrates.