Adult respiratory distress syndrome (ARDS) is a serjous complication of trauma and sepsis. We have earlier shown naloxone, an opiate antagonist, and eyproheptadine, an antiserotonin drug, to be effective in reducing pulmonary platelet trapping (PPT), which is thought to play an important role in the evolution of ARDS in endotoxin-shocked dogs. Endorphins are implicated as pathophysiologic factors in shock, and serotonin is a possible mediator of their action. The present study shows naloxone and cyproheptadine to be equally effective in protecting against PPT in dogs subjected to trauma, and when naloxone is given before the trauma it also obviates the hypotension associated with trauma. In addition, the naloxone- and cyproheptadine-treated animals did not show the increased platelet aggregability usually seen in traumatized dogs.