Exposure to stress can induce analgesia in the formalin test for tonic pain. Here, we report that blockade of opioid receptors by bilateral infusions of naltrexone methylbromide (NMB) (0.1 microgram/0.5 microliter/side) in the ventral tegmental area (VTA) in the midbrain can reduce stress-induced analgesia in this test. These findings indicate that endogeneous opioids act in the VTA to mediate stress-induced analgesia in tonic pain.