The influence of hypolipidemic drug clofibrate on the activity of aldehyde dehydrogenase with different substrates was studied in subcellular fractions of rat liver homogenate. It was shown that under the action of clofibrate the content of the enzyme was increased 2-3-fold in purified peroxisomal fraction as well as in microsomes and mitochondria. No difference was found in the cytoplasmic fraction. Partial purification of clofibrate-induced aldehyde dehydrogenase from microsomes was undertaken. The enzyme is apparently membrane-bound. It has a molecular weight of 187,000 and a subunit size of 47,000, indicating that the molecule is a tetramer. An induced aldehyde dehydrogenase is active with several aliphatic and aromatic aldehydes but not with formaldehyde and glyceraldehyde. The enzyme has Km-values in the millimolar range for acetaldehyde, propionaldehyde, benzaldehyde and phenylacetaldehyde and in the micromolar range for nonanal. Both NAD and NADP serve as coenzymes for the purified aldehyde dehydrogenase. According to substrate specificity, kinetic and molecular properties clofibrate-induced aldehyde dehydrogenase appears to be identical to normal liver microsomal enzyme.