Twenty-eight derivatives of histidine were tested as inhibitors of histidine decarboxylase from rat stomach. Potent inhibition was observed with the methyl ester of L-histidine (1) which had a Ki = 1.8 X 10(-6) M. Substitution on 1 at various positions resulted in a decrease or complete loss of inhibition. Substituent requirements for inhibition of histidine decarboxylase are discussed. A simple disposable apparatus for the radiometric assay of histidine decarboxylase is described.