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The present study was undertaken to investigate the biochemical characteristics of the opioid receptors and opioid peptides in the jerboa (Jaculus orientalis) brain, a sub-desert rodent of Morocco. We have demonstrated the presence of delta, mu and kappa sites in the jerboa brain. The endogenous opioid peptides methionine-enkephalin, beta-endorphin, and dynorphin were evaluated in different physiological states of the animal (active and hibernating). The circulating methionine-enkephalin in different states of the animal (active, hibernating, exposure to cold conditions, and tasting) was evaluated in the plasma, Our results indicate that in the hibernating state the opioid receptors level decreased, whereas the concentration of opioid peptides increased. These findings suggest that both opioid receptors and opioid peptides could be involved in the adaptation of the jerboa to survive under thermal stress. (C) 1997 Elsevier Science Inc.