During previous studies on the inhibition of hydroxyindole‐O‐methyltransferase, several N‐acyltryptamines have been found to be good inhibitors of this enzyme. Substitution of the benzyl or phenyl moiety of N‐phenylacetyltryptamine or N‐benzoyltryptamine with halogen atoms further enhanced the inhibitory activity. Among all the halogen‐substituted inhibitors, the 3,4‐dichloro substitution offered the highest activity. An increase in inhibition of the enzyme was also observed when a fluorine or bromine atom was placed on C‐5 position of the indole nucleus. A combination of the 5‐bromo and 3,4‐dichlorobenzoyl substitutions resulted in the most active inhibitor. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company