Homoveratrylamine inhibited hog kidney diamine oxidase “in vitro”, developed a catatonia-like state in mice and increased vascular permeability of rats. The last two effects were conspicuously enhanced if homoveratrylamine was given together with histamine. The effects of the latter drug on rat blood pressure and on the guinea pig ileum, however, were not modified by previous administration of homoveratrylamine. The possibility that a relationship might exist among homoveratrylamine, histamine and catatonia is discussed.