Polydrug abuse is a common pattern of drug abuse which involves simultaneous or sequential use of multiple drugs. From an economic perspective drugs are commodities, the drug user is a consumer, drugs of abuse have a price, and drug consumption varies with price. Thus, behavioral economic concepts such as unit price, elasticity, and demand may be useful in determining drug use patterns in polydrug environments. These studies will (1) investigate human drug choices in a controlled polydrug laboratory environment and (2) determine whether behavioral economic concepts describe the observed patterns of drug consumption. In each study participants will be randomly assigned to GROUP 1: PRIMARY DRUG VS VEHICLE ALTERNATIVE or GROUP 2: PRIMARY DRUG VS DRUG ALTERNATIVE. Participants in GROUP 1 will have the option of procuring small doses of drug or vehicle by pressing a button. Their counterparts in GROUP 2 will have the option of procuring either of two concurrently available drugs. STUDY 1 will examine methadone consumption when vehicle (GROUP 1) or ethanol (GROUP 2) are concurrently available. STUDY 2 will examine methadone consumption when vehicle and hydromorphone are the concurrent drug alternatives. STUDY 3 will examine intranasal cocaine consumption when vehicle or oral cocaine are the concurrent drug alternatives. STUDY 4 will examine intranasal cocaine consumption when vehicle or ethanol are the concurrent drug alternatives. Price for procuring drug will be increased systematically for participants in both experimental groups. This permits determination of demand curves, elasticity of demand, and cross-price elasticity. Demand will be assessed by observing the effect of price increases of primary drug on primary drug consumption when vehicle is concurrently available (GROUP 1). Elasticity will be assessed by comparing primary drug consumption across price in the presence vehicle or alternative drug (comparison of GROUP 1 and 2). Cross-price elasticity will be assessed by observing the effects of primary drug price on consumption of alternative drug (GROUP 2). These studies will collect economic data on polydrug abuse, test behavioral economic concepts of drug choice, and extend economic principles to the understanding and control of polydrug abuse.