Decision making refers to the process by which subjects choose between competing courses of action based on the expected costs and benefits of their consequences. Lesion studies in rats suggest that the anterior cingulate cortex and the nucleus accumbens are key structures of a neural system that subserves effort-based decision making. Little is known about brain activation associated with effort-based decisions in intact rats. Using an open hypothesis approach, we used 2-deoxy-2[(18)F]fluoro-D-glucose positron emission tomography (FDG-PET) to assess regional metabolic changes in two conditions of an effort-based decision making task. In the "same effort" condition, male rats could choose between two response options associated with the same effort but different reward sizes, i.e., decision making was simply a function of reward size. By contrast, in the "different effort" condition, an integration of different efforts and reward sizes associated with the two response options was necessary before making a decision. Separate PET scans were performed from each condition. Subtractive analysis revealed that metabolic activity was increased in the different effort relative to the same effort condition in the left anterior cingulate, left orbitofrontal and prelimbic cortex region. Metabolic activity was decreased in the infralimbic cortex and septum region. Our findings suggest that making decisions on how much effort to invest to obtain greater rewards evokes changes of metabolic activity in multiple brain areas associated with cognitive, limbic, motor and autonomic functions. This study demonstrates that FDG-PET provides a tool to determine in rats regional brain metabolic activity in cognitive tasks.