Ethanol non-drinker (UChA) and drinker (UChB) rat lines derived from an original Wistar colony have been selectively bred at the University of Chile for over 70 generations. Two main differences between these lines are clear. (1) Drinker rats display a markedly faster acute tolerance than non-drinker rats. In F2 UChA × UChB rats (in which all genes are ‘shuffled’), a high acute tolerance of the offspring predicts higher drinking than a low acute tolerance. It is further shown that high-drinker animals ‘learn’ to drink, starting from consumption levels that are one half of the maximum consumptions reached after 1 month of unrestricted access to 10% ethanol and water. It is likely that acquired tolerance is at the basis of the increases in ethanol consumption over time. (2) Non-drinker rats carry a previously unreported allele of aldehyde dehydrogenase-2 (Aldh2) that encodes an enzyme with a low affinity for Nicotinamide-adenine-dinuclectide (NAD+) (Aldh22), while drinker rats present two Aldh2 alleles (Aldh21 and Aldh23) with four- to fivefold higher affinities for NAD+. Further, the ALDH2 encoded by Aldh21 also shows a 33% higher Vmax than those encoded by Aldh22 and Aldh23. Maximal voluntary ethanol intakes are the following: UChA Aldh22/Aldh22 = 0.3–0.6 g/kg/day; UChB Aldh23/Aldh23 = 4.5–5.0 g/kg/day; UChB Aldh21/Aldh21 = 7.0–7.5 g/kg/day. In F2 offspring of UChA × UChB, the Aldh22/Aldh22 genotype predicts a 40–60% of the alcohol consumption. Studies also show that the low alcohol consumption phenotype of Aldh22/Aldh22 animals depends on the existence of a maternally derived low-activity mitochondrial reduced form of nicotinamide-adenine-dinucleotide (NADH)-ubiquinone complex I. The latter does not influence ethanol consumption of animals exhibiting an ALDH2 with a higher affinity for NAD+. An illuminating finding is the existence of an ‘acetaldehyde burst’ in animals with a low capacity to oxidize acetaldehyde, being fivefold higher in UChA than in UChB animals. We propose that such a burst results from a great generation of acetaldehyde by alcohol dehydrogenase in pre-steady-state conditions that is not met by the high rate of acetaldehyde oxidation in mitochondria. The acetaldehyde burst is seen despite the lack of differences between UChA and UChB rats in acetaldehyde levels or rates of alcohol metabolism in steady state. Inferences are drawn as to how these studies might explain the protection against alcoholism seen in humans that carry the high-activity alcohol dehydrogenase but metabolize ethanol at about normal rates.