This research deals with behaviors that are part of drug dependence. The purpose is to study how certain factors, including money, the amount of drug available, and the amount of work effort, affect cocaine drug choice. Specifically, we will examine the effects of two issues/factors. The first is how hard individuals are willing to work to obtain a drug; the second is how much drug would individuals choose instead of money, when the amount of probability of money is predictable or unpredictable.