This project is the first of a series of studies to evaluate the conditions under which drugs develop conditioned reinforcing properties. It is thought that these conditioned reinforcing properties are influential in the initiation and maintenance of drug-taking behavior and may also influence the subjective effects experienced by the individual. The first study was designed to determine whether placebo capsules that were associated with high rates of reinforcement produced changes in mood and physiological states. The results showed that subjects preferentially chose to ingest the capsule color associated with high reinforcement and they also reported increases in positive subjective states. In a second study, subjects given exposure to capsules associated with high versus low rates of reinforcement did not report increases in the elation and positive mood scales in the absence of reinforcement. These results indicate that there is a differentiation between conditioning of mood states and drug preference. Future studies are designed to evaluate the process of conditioned reinforcement and the separation of this process and alterations in mood states.