Aldehyde dehydrogenase (AHD) exists as isozymes which are differentially distributed among tissues and subcellular fractions of mouse tissues. Genetic variants for liver mitochondrial (AHD-1) and cytoplasmic (AHD-2) isozymes have been used to map the responsible loci (Ahd-1 and Ahd-2) on chromosomes 4 and 19 respectively. Evidence for a regulatory locus (Ahd-3r) controlling the inducibility of the mouse liver microsomal isozyme (AHD-3) has also been obtained. More recent studies have described genetic and biochemical evidence for three additional AHD isozymes: a stomach isozyme (AHD-4); another liver mitochondrial enzyme (AHD-5); and a testis isozyme (AHD-6). Genetic analyses have indicated that AHD-4 and AHD-6 are encoded by distinct but closely linked loci on the mouse genome (Ahd-4 and Ahd-6), which segregate independently of Ahd-1 and Ahd-2. Liver mitochondrial isozymes, AHD-1 and AHD-5, have been purified to homogeneity using affinity chromatography. The very high affinity of AHD-5 for acetaldehyde suggests that this enzyme is predominantly responsible for acetaldehyde oxidation in mouse liver mitochondria.