To provide a molecular basis for understanding the possible mechanism of action of antidipsotropic agents in laboratory animals, aldehyde dehydrogenase (ALDH) isozymes were purified and characterized from the livers of hamsters and rats and compared with those from humans. The mitochondrial ALDHs from these species exhibit virtually identical kinetic properties in the oxidation and hydrolysis reactions. However, the cytosolic ALDH of human origin differs significantly from those of the rodents. Thus, for human ALDH-1, the K_m value for acetaldehyde is 180 ± 10 μM, whereas those for hamster ALDH-1 and rat ALDH-1 are 12 ± 3 and 15 ± 3 μM, respectively. K_m values determined at pH 9.5 are virtually identical to those measured at pH 7.5. In vitro human ALDH-1 is 10 times less sensitive to disulfiram inhibition than are the hamster and rat cytosolic ALDHs. Competition between acetaldehyde and aromatic aldehydes or naphthaldehydes for the binding and catalytic sites of ALDHs shows their topography to be complex with more than one binding site. This also follows from data on substrate inhibition and activation, effects of NAD⁺ on ALDH-catalyzed hydrolysis of p-nitrophenyl esters, substrate specificity toward aldehydes and p-nitrophenyl esters, and inhibition by disulfiram in relation to oxidation and hydrolysis catalyzed by the ALDHs. The data further suggest that acetaldehyde cannot be considered as a “standard” ALDH substrate for studies aimed at aromatic ALDH substrates, e.g. biogenic aldehydes. Apparently, in human liver, only mitochondrial ALDH oxidizes acetaldehyde at physiological concentrations, whereas in hamster or rat liver, both the mitochondrial and cytosolic isozymes will do so.