The "intentional stance" is the disposition to treat an entity as a rational agent, possessing particular beliefs, desires, and intentions, in order to interpret and predict it's behavior. The intentional stance is a component of a broader social cognitive function, mentalizing. Here we report a study that investigates the neural substrates of "on-line" mentalizing, using PET, by asking volunteers to second-guess an opponent. In order to identify brain activity specifically associated with adoption of an intentional stance, we used a paradigm that allowed tight control of other cognitive demands. Volunteers played a computerised version of the children's game "stone, paper, scissors." In the mentalizing condition volunteers believed they were playing against the experimenter. In the comparison condition, volunteers believed they were playing against a computer. In fact, during the actual scanning, the "opponent" produced a random sequence in both conditions. The only difference was the attitude, or stance, adopted by the volunteer. Only one region was more active when volunteers adopted the intentional stance. This was in anterior paracingulate cortex (bilaterally). This region has been activated in a number of previous studies involving mentalizing. However, this is the first study suggesting a specific link between activity in this brain region and the adoption of an intentional stance.