The hypothesis that the animal model of inescapable shock (IES) is an appropriate model for posttraumatic stress disorder (PTSD) predicts that re-exposure to a traumatic stressor will precipitate opioid-mediated stress-induced analgesia in people with PTSD. Eight Vietnam veterans with PTSD and eight matched veterans without PTSD viewed a combat videotape under naloxone and placebo conditions in a randomized double-blind crossover design. In the placebo condition, but not after naloxone, the PTSD subjects reported a 30 percent decrease in pain intensity ratings of standardized heat stimuli after the combat videotape. Point biserial correlations revealed that change in pain perception was the most highly correlated with PTSD of all variables tested, including biochemical, physiological, and self-report. These results suggest that a centrally mediated opioid response to traumatic stimuli is an important feature of PTSD. Possible implications of this finding for the psychobiology of PTSD are discussed.