In decision under risk, people choose between lotteries that contain a list of potential outcomes paired with their probabilities of occurrence. We previously developed a method for translating such lotteries to mathematically equivalent "motor lotteries." The probability ofeach outcomeina motor lotteryis determinedby the subject's noiseinexecuting amovement.Inthis study,weused functional magnetic resonance imaging in humans to compare the neural correlates of monetary outcome and probability in classical lottery tasks inwhich informationaboutprobabilitywas explicitlycommunicatedtothe subjectsandinmathematicallyequivalentmotorlotterytasks in which probability was implicit in the subjects' own motor noise.Wefound that activityinthe medial prefrontal cortex (mPFC) and the posterior cingulate cortex quantitatively represent the subjective utility of monetary outcome in both tasks. For probability, we found that the mPFC significantly tracked the distortion of such information in both tasks. Specifically, activity in mPFC represents probability information but not the physical properties ofthe stimuli correlated with this information. Together, the results demonstrate that mPFC represents probability from two distinct forms of decision under risk.