Using the principles of operant conditioning, variables will be studied that determine the development, functioning and elimination of drugs as reinforcers via the oral route. Subjects will be rats and rhesus monkeys. Drugs serve as reinforcers when self-administered at rates and volumes in excess of vehicle (usually water) values. When a drug functions as a reinforcer, it increases the frequency of behaviors that result in its presentation. Methods used in analyzing other reinforcers may be applied to drugs. Variables that affect both whether and to what extent a drug is a reinforcer may be categorized as history factors (e.g., prior drug experience), current circumstances (e.g., food deprivation) and response consequences (e.g., drug concentration). These variables will be studied parametrically, i.e., across a range of values, and interactions among variables will be explored systematically. The objectives of this research are (1) to investigate functional relations between certain independent variables (e.g., prior drug-taking experience) and several dependent variables (e.g., response rate maintained by drug presentation, volume of drug solution consumed); (2) to study interactions among these independent variables (e.g., both drug concentration and reinforcement schedule parameter will be varied); (3) to assess the relative importance of the variables studied, and (4) to demonstrate that tolerance and physiological (physical) dependence can develop under certain conditions when drugs function as reinforcers via the oral route. These objectives will have three consequences. One will be the establishment of an infrahuman model of oral drug dependence. A second consequence will be the development of a systematized body of knowledge that may be used in the analysis of the complicated phenomena of human drug dependence. A third consequence will be the specification of methods for establishing drugs as reinforcers via the oral route for infrahuman primates.