Two aldehyde reductases (EC 1.1.1.2), I and II, have been isolated from pig liver. Both are monomeric (Mr approximately equal to 35 000) and NADPH-dependent. Their activity is inhibited by barbiturates. The enzymes reduce essentially aromatic aldehydes, with a preference for those bearing an electron-withdrawing group in the para position. Substrates with a carboxyl group are specially good substrates for reductase I. This may indicate the presence of a positively charged group in the substrate binding site. The binding of NADPH to reductase I causes a red shift of the coenzyme absorption; this shift is characteristic of B-stereospecific dehydrogenases. Nevertheless, this is not confirmed by the stereochemical study with labelled NADPH. The pro-R hydrogen of NADPH is transferred to the re face of the aldehyde. The stereochemical course of reductase I is identical to that of liver alcohol dehydrogenase, but the two enzymes differ by the absence of Zn and of reactive thiol in reductase I, and by the action of pyrazole on the activity. Considerable differences in substrate specificity and immunological properties have been found between reductase I and II but reductases I from liver of different species have some relationship. Reductase I from pig brain and pig kidney seem to be identical to reductase I from pig liver.