We have developed an innovative use of positron emission tomography that has broad applications in drug development. Based on the FDG method (Phelps et al. 1979) for assessing glucose metabolism, the method requires rigorous experimental control of subjects and standardized data acquisition and analyses. In localizing net metabolic drug effects, measured in quantifiable terms, we have derived a new conceptual basis for examining pharmacologically induced changes in brain function and a new model for predicting drug effectiveness. We applied this method to studies of drugs in three different classes and noted marked differences in distribution and magnitude of metabolic effects. This approach presents an opportunity to selectively examine measurements of the glucose metabolic changes induced by specific pharmacological probes on intermediary metabolic pathways, including regulation of gene expression and the metabolic consequences of neurotransmitter alteration in pharmacologically targeted neuronal systems. Development of these methods provides new approaches for studying neurobiological mechanisms, and can contribute significantly to the process of new drug development.