Two experiments tested the effects of opioid receptor blockage on behavior. In Experiment 1, rats reinforced for lever pressing with either sucrose or food pellets received treatment with saline, 2, and 10 mg/kg naloxone, i.p. (within-subject design). Naloxone 10 mg/kg increased response latency, but 2 mg/kg had no effect. When shifted to extinction (between-group design), naloxone (2 and 10 mg/kg) facilitated extinction relative to saline animals, after reinforcement with either sucrose or food pellets. In Experiment 2, after 10 sessions of access to 32% sucrose or an empty tube (between-group design), all rats were exposed to the empty tube while allowing them to jump over a barrier into a different compartment. Escape latencies were shorter for downshifted saline than for saline rats always given access to the empty tube. This escape-from-frustration effect was eliminated by naloxone (2 mg/kg, i.p.). Opioid blockage appears to reduce the value of alternative incentives. © 2009 Elsevier Inc. All rights reserved.