One of the first observations of drug dependence and addictive behaviour in a nonhuman species was made in morphine-dependent chimpanzees. The chimpanzees had to make a choice between food and a morphine injection syringe that they remembered from earlier morphine infusions given by the experimenter. When the chimpanzees were deprived of morphine, they would choose the morphine injection syringe, demonstrating their dependence on this drug (Spragg 1940). In the 1960s, drug self-administration paradigms with fully automated intravenous infusions were developed for rats and monkeys. In these instrumental learning procedures, animals were trained to self-administer drugs of abuse by leverpressing or nose-poking (Weeks 1962;Thompson and Schuster 1964). It was shown that laboratory animals readily self-administer the same addictive drugs that are used by humans, including cocaine, amphetamine, nicotine, heroin and morphine. This led to the hypothesis that the rewarding effects of drugs are a pharmacological property, rather than from involvement of psychological and social processes that at time were predominantly thought to predispose to drug addiction.