Two-bottle preference tests can be conducted in many ways, but the most common is to offer animals one water bottle and one bottle with an alcohol solution, usually 10%, a little less than most table wines (McClearn, Rodgers 1959;Mardones, Segovia-Riquelme 1983). Access is most often continuous, and the amount drunk from each bottle is measured each day. Intake is expressed as (g alcohol) / (kg body weight), or as the proportion of total fluid taken from the alcohol bottle (preference ratio). To measure withdrawal, animals are made dependent. A low grade of dependence can be induced by a single (or repeated) high-dose bolus injection or intragastric infusion of alcohol (McQuarrie, Fingl 1958;Majchrowicz 1975). To achieve more severe dependence, alcohol is administered chronically in a liquid diet (Tabakoff et al. 1977) or the animal is confined to a sealed chamber filled with alcohol vapor for a period of days (Goldstein, Pal 1971). Recently, interest has increased in exposing animals to chronic, intermittent periods of vapor, which increases the intensity of withdrawal (Becker, Lopez 2004). Dependence cannot be measured directly, but must
