Three multiforms of NADPH-dependent aldehyde reductase have been purified to homogeneity from chicken kidney. The enzymes were monomeric proteins with similar molecular weights around 39 000 but with different pI values. Two of them exhibited, almost the same heat stability, a broad optimal pH of 6-6.5 and preference for NADPH as a cofactor. They were high-Km aldehyde reductases which reduced various aldehydes, hexonates, alpha-diketones, xylose, glucuronolactone and ethyl acetoacetate, and which were inhibited to similar degrees by heavy metal ions, organic acids, p-chloromercuribenzoate, indomethacin and phenobarbital. The third aldehyde reductase was distinct from the other two enzymes in its heat lability, sharp acidic pH optimum of 6.0, and dual cofactor specificity. The enzyme showed low Km values for the above-mentioned substrates except hexonates and 4-carboxybenzaldehyde, and it specifically reduced lauryl aldehyde, acetoin and ketosteroids. This enzyme was less sensitive to the above-mentioned inhibitors than the high-Km aldehyde reductases, but the enzyme was moderately inhibited by chlorpromazine and was activated about 1.5 times by the addition of sulfate ions. It was demonstrated using antibodies against one of the high-Km forms and the low-Km form of aldehyde reductase that the two high-Km forms were immunologically identical with each other but not with the low-Km form, and that the high-Km and low-Km forms were widely distributed in various tissues of chicken.