A, B. Trial-averaged lever presses per 2-min period averaged across both levers during the baseline periods compared to pressing during the CS separated for presses on the lever that, in training, delivered the same outcome as predicted by the CS (CS-Same) and pressing on the other available lever (CS-Diff) for the delta- (A) or mu-opioid receptor antagonist (B) group. C, D. Trial-averaged entries into the food-delivery port during the baseline and CS periods for the delta- (C) or mu-opioid receptor antagonist (D) group. Error bars ±s.e.m. *P<0.05, **P<0.01, ***P<0.001.