In this work we demonstrate that the stress-induced reduction in bone marrow granulocyte-macrophage colonies, reported previously from our laboratory, is prevented by both the inhibition of the hypothalamic-pituitary-adrenal axis (HPAA) and the blockage of opioid receptors. The inhibition of the HPAA was obtained through the administration of dexamethasone (1 mg/kg). The blockage of opioid receptors was done in two ways, by the administration of naltrexone (8 mg/kg) and induction of tolerance to morphine. On the other hand, no protection was observed in metyrapone treated rats. We suggest that the two physiological systems, opioid and HPAA, mediate the stress-induced myelosuppression and that these systems may function independently in this particular situation. (C) 1997 International Society for Immunopharmacology.194227233