I. On the basis of kinetic properties and sensitivity to pyrazole inhibition, it is shown that li~er alcohol dehydrogenase present in human mainly corresponded to class I and in rat to class ADH-3 which differed in a number of parameters. _ 2. Two different aldehyde dehydrogenase (ALDH) isoenzymes were detected m both human and rat liver. The human isoenzymes corresponded to the ALDH-1 and ALDH-11 trpe. _ _ 3. In the rat one isoenzyme had low Km and showed similar activity than m human hver ~ut differed in their sensiti;ity to both disulfiran and nitrofazole inhibition whereas the other presented high Km and showed greater activity than the human one. 4. Caution must be therefore paid when extrapolating to human subjects the data on ethanol metabolism obtained with rats.