> Despite the repeated claims of METH being more addictive or preferred than AMPH, proven differences between METH and AMPH in addiction liability and in reward efficacy have evaded researchers. Animals self-administer METH and AMPH at comparable rates (Balster and Schuster 1973) and humans prefer similar doses (Martin et al. 1971). Also, neither humans nor animals discriminate between equal doses of METH and AMPH (Huang and Ho 1974; Kuhn et al. 1974; Lamb and Henningfield 1994). Furthermore, while METH is commonly believed to be a more potent central psychostimulant than AMPH, no direct comparison on the potency of the two drugs to stimulate central processes have been verified. In addition, no previous study has directly compared the acute effects of the two drugs on locomotor activity, an important central process that contributes tothe definition of psychostimulant. Moreover, there are no known neurobiological differences in action between METH and AMPH that would account for the putatively greater addictive, rewarding, or psychomotor properties of METH.