The enzyme dopamine-β-oxidase not only effects the hydroxylation of dopamine to norepinephrine, but accepts as substrates a wide vareety of phenethylamine derivatives such as epinine, m-tyramine and their branched α-methyl derivatives, m-methoxytyramine, which is converted to normetanephrine, 3,5-dimethoxytyramine, and to some extent even mescaline. The fact that many well-known sympathomemetic drugs are good substrates forthe enzyme raises the pissibility that compounds such as amphetamine, paredrinol, paradrine and α-methyl- m-tyramine owe some of their activity to the corresponding metabolites which are all derivatives of ephedrine. The non-specificity of dopamine-β-oxidase prompted a search for competitive or specific inhibitors. enzylhydrazine, isosteric with phwnethylamine, inhibited the enzyme strongly at concentrations of 10 −5 M. The therapeutic and clinical implications of this observation are pointed out.