Properties of the phenobarbital induced cytoplasmic aldehyde dehydrogenase (EC 18.104.22.168) have been studied in rat liver. 7-12-Fold higher levels were seen in the cytoplasmic activities after phenobarbital treatment in reactor compared to non-reactor animals with high concentrations of acetaldehyde (18 mM) and propionaldehyde (9 mM). No difference was found with 0.12 mM acetaldehyde, 2 mM glycolaldehyde, 6 mM formaldehyde or 0.5 mM betaine aldehyde. The reactor group also had slightly higher activity in the mitochondrial fraction with the high acetaldehyde and propionaldehyde concentrations. In the microsomal fraction, the activities showed no differences at any substrate concentration. An induced aldehyde dehydrogenase was purified 70-fold by chromatographic techniques. It had different molecular and enzymic properties than the main high-Km enzyme normally present in rat liver cytoplasm. The pI of the induced enzyme was about 7.0 as measured by isoelectric focusing. It was active with several aliphatic and aromatic aldehydes but not with formaldehyde, glycolaldehyde or D-glyceraldehyde. The Km-values for propionaldehyde and acetaldehyde were in the millimolar range. Millimolar concentrations of aromatic aldehydes caused a strong substrate inhibition. The enzyme was inhibited by submicromolar concentrations of disulfiram. Estrone, deoxycorticosterone, progesterone and diethylstilbestrol also affected the enzyme activity.