For decades, researchers have used animal self-administration models to examine the effects drugs of abuse have on physiology and behavior. Sophisticated self-administration procedures have been developed to model many different aspects of drug addiction. The hold-down procedure provides animals with control over the amount of each injection. Holding the lever down turns the syringe pump on and subsequently releasing the lever turns the pump off. In this way, animals can hold the lever down for any duration of time thereby self-administering any dose on a continuous spectrum. This procedure eliminates some of the ambiguity in translating results from effects only observed at one unit dose and allows examination of which dose the animal "prefers" at different times.