Michaelis constants and maximal velocities for phenylacetaldehyde (a metabolite of phenylethylamine), 3,4-dihydroxyphenylacetaldehyde (a metabolite of dopamine), 5-hydroxyindole acetaldehyde (a metabolite of serotonin), and 3,4-dihydroxyphenylglycolaldehyde (a metabolite of epinephrine and norepinephrine) have been determined for both cytoplasmic (E1) and mitochondrial (E2) isozymes of human liver aldehyde dehydrogenase (EC 1.2.1.3). Kinetic constants with biogenic aldehydes have never been previously determined for individual homogeneous isozymes of aldehyde dehydrogenase from any species. Mathematical treatment of these constants suggests that competition with acetaldehyde during alcohol metabolism would severely inhibit dehydrogenation of biogenic aldehydes with the mitochondrial and not the cytoplasmic isozyme of human liver aldehyde dehydrogenase.