In this study, Volkow and her team used positron emission tomography (PET) to explore the effects of oral doses of methylphenidate on dopamine in the brains of 11 healthy young subjects. In essence, they gave the subjects a placebo, and then a radioactive substance that competed, in their brains, with endogenous dopamine for dopamine receptors. They measured the level of this radioactive substance in the subjects’ brains. Sometime later they gave the subjects methylphenidate, then the radioactive substance, and once again measured the level of the substance in the subjects’ brains. Then they compared the level of the radioactive substance given under the influence of the placebo with that given under the influence of methylphenidate. They found that the level was lower under the influence of the drug than under that of the placebo. This lower level of the radioactive substance in the drug state suggested that there was more dopamine in the subjects’ brains under the influence of methylphenidate than a placebo.