Cocaine abuse continues to be an important public-health concem. Because cocaine abuse is not an automatic consequence of cocaine use, it is important to study factors that may lead from the latter to the former. Two such factors that have been implicated in cocaine abuse (and in the abuse of other drugs, as well) are tolerance and sensitization that can develop as a result of repeated exposure to the drug. Overwhelming evidence now indicates that behavioral factors can playa role in whether, and the degree to which, tolerance or sensitization occurs. This application is to request continued support for research on behavioral factors involved in the development of tolerance and sensitization to behavioral effects of cocaine. Specifically, the research will have two major aims. The first is to expand the analysis of the way in which ostensibly subtle differences in how consequences of behavior occur modulate the development of tolerance and/or sensitization to effects of cocaine on that behavior. The second is to examine a potential behavioral mechanism, sensitivity to startle, of cocaine's effects on a model of purposive behavior that reveals tolerance to the drug's effects.