AbstractThe hydrazides of both optical isomers of amino acids and isolysergic acid were tested for effects on amine oxidases. There was little action on monoamine oxidase (MO). However, some of the amino acid hydrazides, such asL‐leucylhydrazide, turned out to be very effective inhibitors of diamine oxidase (DO). In every instance there was a significant difference between the action of the two antipodes. In seven pairs of antipodal amino acid derivatives, theL‐form was much more effective than theD‐form. Some conclusions about the optical configuration of the reactive site of DO have been drawn. The possible relationship between the low efficiency of theD‐amino acid isomers as inhibitors and their low toxicity, as reported in the literature, has been di