Data exist which show clearly that the behavioral effects of repeated administration of abused drugs can depend on the behavioral situation. The most common outcome of repeated administration of abused drugs is the development of tolerance. In the proposed project animal subjects will be used to examine four major possible determinants of tolerance to behavioral effects of abused drugs. Specifically, the four questions that will be addressed are: (1) Is tolerance to the effects of abused drugs on schedule-controlled behavior dependent on drug-produced changes in reinforcement frequency? (2) Is tolerance to drug effects on schedule-controlled behavior dependent on baseline response rates? (3) Is tolerance to disruptions of stimulus control of responding dependent on the "complexity" of the behavioral task (i.e., on the baseline levels of stimulus control)? (4) Is tolerance to effects of abused drugs on schedule-controlled behavior dependent on the type of event (e.g., food presentation, shock avoidance, conditioned reinforcers) maintaining the behavior? In the experiments proposed here animal subjects will be trained to make simple manipulative responses in highly controlled situations. The simple responses will be used as models for more complex units of behavior, in the hope that principles derived from well controlled, simple situations can be used to predict behavior in more complex settings.