of the first researchers to create a model of chronic morphinism in a chimpanzee to explore the role of operant conditioning in relation to a drug dependency. When deprived of both food and morphine, chimpanzees would repeatedly attempt to seek out the drug of choice, even doing so much as to physically pull the experimenter into the room housing morphine and syringes. Weeks (1962) published an account of the first true use of the intravenous self-administration paradigm in a study aiming to model morphine addiction in unrestrained rats. For the first time, an addictive substance served as an operant reinforcer and rats self-administered morphine to satiety in stereotyped response patterns. The scientific community quickly adopted the self-administration paradigm as a behavioral means to examine addictive processes and adapted it to non-human primates. Thompson and Schuster (1964) studied the relative reinforcement properties of morphine in restrained rhesus monkeys using intravenous self-administration. Significant changes in response to other types of reinforcers (i.e., food, shock avoidance) were observed in drug-dependent subjects. In 1969, Deneau, Yanagita and Seevers provided macaque monkeys free access to a variety of addictive substances for investigating whether nonhuman primates would voluntarily initiate self-administration of these substances. Initiation and maintenance of self-administration produced dependence and toxicity in monkeys, thereby more closely approximating important aspects of drug addiction in humans and allowing for the first of modern self-administration studies. The procedure of testing the efficacy of a pharmacological agent as a reinforcer would soon become a standard assay. Most frequently, studies were performed in nonhuman primates to identify addictive potential, as required by the drug development process. In 1983, Collins et al. published a landmark paper in which rats were exposed to a battery of 27 psychoactive substances. The team compared test drug self-administration rates with saline vehicle self-administration rates. If animals self-administered at a rate significantly greater than vehicles, the drug was considered an active reinforcer with addictive potential. With few exceptions, the abuse liability observed in rats paralleled that observed from previous research in monkeys. In light of these similarities between the different animal models, it was identified that the addictive potential of psychoactive substances could be investigated using rats instead of nonhuman primates. Technique Acquisition Operant conditioning represents the behavioral paradigm underlying self-administration studies. Although not always required, subjects may be first pre-trained to perform some action, such as a lever press or nose poke to receive a