Odors released by stressed rats produce opioid analgesia in unstressed rats.

When unstressed rats were placed in a chamber containing the odor of a conspecific that had received electric shock, they became analgesic as assessed by the formalin test. The odors of nonstressed conspecifics and novel odors produced no such effect. This analgesia was reversed by naltrexone. The data suggest that naturalistic stimuli innately associated with aversive environmental events can activate endogenous pain control mechanisms.