|Abstract||Previous research has shown that the opioid system is engaged during surprising reward loss events. Frustration theory predicts that the opioid system's modulatory influence on such situations is attributable to three potential mechanisms: comparison of received and expected rewards, intensity of the frustrative emotional response induced by this comparison, or consolidation of the frustration memory. Four experiments provide no supportive evidence for the hypothesis that the opioid system participates in the consolidation of the frustration memory. These experiments involved situations varying in terms of the type of reward reduction (complete or partial), the type of reward (solid food pellets or sucrose solutions), and the type of behavior system (anticipatory or consummatory behavior). A fifth experiment suggests that opioids distort the comparison between expected and received rewards, narrowing the possible opioid mechanisms to modulation of incentive comparison and/or the intensity of primary frustration.