Ethanol or ethyl alcohol is a molecule that, in mammals, is naturally present at low concentrations due to its production by gastrointestinal flora fermentation activity. However, it is remarkable that this metabolite, with a clearly minor role in regular vertebrate metabolism, can be oxidized into acetaldehyde through several enzymatic and non-enzymatic mechanisms, which comprise the activity of more than ten enzymes and isozymes, many of them broadly distributed in different species and tissues. In correspondence, acetaldehyde can also be oxidized into acetate through several enzymatic pathways that involve about ten enzymes and isozymes which also have a broad distribution. In this article, a complete review of the aforementioned metabolic pathways is elaborated. From this group, the participation and wide distribution of alcohol dehydrogenase and aldehyde dehydrogenase systems are emphasized. The mechanisms of reaction, kinetic characteristics and physiological relevance are described, and finally, the possible physiological role of these enzymatic systems as responsible to synthesize or catabolize several endogenous metabolites that regulate growth, metabolism, differentiation and neuroendocrine function in mammals are discussed.