Our previous findings that the degree of endotoxin-induced hypotension in the dog is inversely related to ambient temperature (19 degrees through 30 degrees C) and that only increased doses of naloxone are effective at 19 degrees C suggested that opioid activity is also influenced by ambient temperature, increasing in the cold and decreasing in the warm. Others have reported increases in plasma beta-endorphin in rats with acute exposure to both 5 degrees and 36 degrees C. In this study we measured changes in pain thresholds after both acute and chronic exposures to lesser alterations in ambient temperature as a potentially more sensitive index of changes in central opioid activity. Compared to 24 degrees C there was a marked increase in pain threshold with acute exposure to 10 degrees C and marked decreases at 30 degrees and 35 degrees C. A slight decrease occurred after 30 minutes but not 60 or 120 minutes at 19 degrees C. All acute changes disappeared three hours after the animals had been returned from the altered ambient temperature to 24 degrees C. No changes were observed after six days chronic exposure to 10 degrees or 30 degrees C. These findings suggest that moderate, acute changes in ambient temperature can produce inversely related, adaptable alterations in central opioid activity.