Drug tolerance can be broadly defined as a reduction in the effects of a drug as a consequence of repeated exposure to that drug. This is shown as a shift to the right of the dose-response curve (DRC) of a drug, meaning that a higher dose of the drug is required to produce the same degree of effect in a tolerant individual as that seen in a naive subject (Kalant et al., 1971 ). The field of drug tolerance has been studied by investigators with a variety of interests, ranging from clinical pharmacological problems to tolerance as a form of biological plasticity. It is, however, in the study of drugs of abuse that the phenomenon of tolerance has been most extensively examined. For example, tolerance to opiates and alcohol has been documented in literature as early as the 19th century (Goudie and Emmett-Oglesby, 1989 ). The main reason for such an interest in tolerance in the field of drug addiction is that it is widely believed to be a critical factor in the acquisition and maintenance of the addictive process (Kalant, 1988 ).