Guided by regulatory focus theory, we examined whether messages tailored to individuals' promotion- or prevention-goal orientation (regulatory focus) elicit positive thoughts and feelings about physical activity and increase participation in physical activity. Inactive participants (N = 206) were assigned randomly to receive either promotion-focused or prevention-focused messages encouraging physical activity. Two weeks after message exposure, we assessed participants' thoughts and feelings about physical activity and physical activity behavior. Tailored messages that fit individuals' regulatory focus led to greater physical activity participation and more positive feelings than non-fit messages, particularly in the promotion-focused condition. Furthermore, positive retrospective feelings about physical activity mediated the effects of the tailored messages on behavior. These findings provide support for regulatory focus theory and direction for enhancing the effectiveness of messages encouraging physical activity and other health behaviors.