Previous studies from our laboratory indicate that acute but not chronic hypoxia decreases the hemorrhage volume required to elicit reflex hypotension. Furthermore, chronically hypoxic animals exhibit an elevated hypotensive threshold during both normoxia and hypoxia compared with control animals. Because reports suggest that opioid and serotonergic mechanisms may be involved in mediating the sympathoinhibition that occurs with hemorrhage, we hypothesized that opioid and/or serotonergic systems are stimulated during hemorrhage under conditions of acute hypoxia and suppressed after chronic exposure to hypoxia and are thus responsible for the altered cardiovascular responses to hemorrhage under each condition. Control and chronically hypoxi rats were administered either the opioid receptor antagonist naltrexone (1 mg/kg), the selective 5-hydroxytryptamine receptor subtype 3 (5-HT3) serotonergic receptor antagonist MDL-72222 (0.5 mg/kg), or their respective vehicles intravenously before hemorrhage was initiated during normoxia or hypoxia (FIO2 = 0.12). In control animals, pretreatment with naltrexone increased the hemorrhage was initiated volume required to achieve hypotension in hypoxic but not normoxic conditions. Naltrexone had no effect on hypotensive threshold in chronically hypoxic animals under conditions of either normoxia or hypoxia. In addition, MDL-72222 had no effect on hypotensive threshold in either control or chronically hypoxic animals in either normoxic or hypoxic conditions. We conclude that endogenous opioids may contribute to the reflex hypotension that occurs during hypoxic hemorrhage in control rats, while no such involvement is evident in chronically hypoxic animals. Furthermore, peripheral 5-HT3 receptors are not likely involved in this response during either normoxic or hypoxic hemorrhage in control or chronically hypoxic rats.