Endogenous opioid peptides have been implicated in stress-induced analgesia and stress-induced feeding behavior. An earlier study from our laboratory showed that rats subjected to cold swim stress consumed significantly more food compared to controls. The present study describes changes in the levels of various opioid peptides in the central nervous system and periphery due to cold swim stress. Male Sprague-Dawley rats were subjected to cold swim stress (1 degree C for 5 min), then sacrificed by decapitation; brain, pituitary, adrenals and plasma were collected. Tissue extracts were assayed for opioid peptides by RIA. Cold swim stress resulted in analgesia which could be blocked by prior administration of naloxone, as observed by a tail-flick latency test. Cold swim stress caused a 42% decrease in pituitary beta-endorphin, but increased the level of this peptide in the hypothalamus and plasma by 36% and 337%, respectively. Dynorphin level decreased by 62% in the hypothalamus, but was not affected in the pituitary. Levels of Leu-enkephalin and Met-enkephalin decreased in the adrenal gland by 37% and 18%, respectively, but were not significantly affected in the CNS. These results indicate that cold swim stress has a differential effect on the level of CNS and peripheral opioid peptides, and that both central and peripheral opioid peptides may be important in stress-induced analgesia and feeding behavior.