The objective of this program is to study the behavioral effects of drugs of abuse and how these drugs interact with each other, particularly during chronic administration. Two different behaviors will be studied, schedule-controlled behavior and motor activity. Experiments will be conducted in three different species, rats, mice, and pigeons, so that the generality of the drug interactions can be determined across species as well as across behaviors. Typically the effects of several drugs will be studied alone and on acute basis, then one of the drugs (interaction drug) will be combined with each of the others to determine the effects of acute drug interactions on behavior. Subsequently, the interaction drug will be given chronically for 4 weeks, usually by placing it in the drinking water. After 4 weeks of chronic administration, dose-effect curves for the effects of the other drugs on behavior will be determined, while the animal continues to drink the interaction drug solution. Next the drug solution is withdrawn. In some instances the dose-effect curves for the other drugs will be redetermined during withdrawal, but generally the dose-effect curves will not be redetermined until 4 weeks until after withdrawal. This design should detect interesting drug interactions before, during and after chronic drug administration. When interesting drug interactions are detected, attempts will be made to study the mechanism of these interactions by measuring any alterations in blood levels of one drug after chronic administration of another, or by administering active metabolites or optical isomers of the drugs in question.