The present study investigated the role of μ, δ, and κ receptors within the RVM in mediating expression of conditional hypoalgesia (CHA). Five groups of rats with RVM cannulae were given daily sessions of paired or unpaired presentations of an auditory CS (white noise) and foot shock across three consecutive days. On the test day, rats in the Paired condition were injected with the μ antagonist CTAP, the δ antagonist naltrindole, the κ antagonist nor-BNI, or saline. Rats in the Unpaired condition were injected with saline. TFLs were measured before and after injections, as well as during and after presentations of the CS. The results showed that none of the drugs affected baseline TFLs. During CS presentation, rats in the Paired condition injected with saline showed longer TFLs than those in the Unpaired condition given saline, confirming the presence of CHA. Expression of this response was blocked by CTAP, but was unaffected by naltrindole or nor-BNI. These results suggest that μ, but not δ or κ, opioid receptors in the RVM mediate expression of CHA.