Publisher Summary Indolealkylamines, which are substrates of monoamine oxidase (MAO), yield corresponding indoleacetic acids as their final metabolic products. The whole process comprises two steps: in the first step, initiated by MAO, the amine undergoes oxidative deamination to the aldehyde, and in the second step, catalyzed by aldehyde dehydrogenase, the intermediate aldehyde is oxidized to the acid. The intermediate indoleacetaldehydes are not stable enough to permit isolation and purification from biological material. Therefore, their presence is established by trapping and converting them in situ into more stable derivatives; their rate of formation may be followed by measuring disappearance of the amine and/or formation of the acid. Indoleacetaldehyde and 5-hydroxyindoleacetaldehyde are completely inactive on isolated smooth muscle preparations and have no modifying effect on the activity of their corresponding amines.