The possible role of the opioid system in the cardiovascular function is an area that attracted significant attention in recent years. In order to clarify the possible role of opioids in cardiac regulation we have studied the myocardial effects of some opioids and their antagonists. Our results show that in vitro some opioids (dermorphin, alpha- and gamma-endorphins, dalargin and other) may exert inotropic, but not chronotropic, action and may act as modulatory agents on the cholinergic myocardial effects. Some of the opioidergic effects were blocked by naloxone and some were not. The results of pharmacological and physiological analysis show that opioidergic-cholinergic interaction may occur at the postsynaptic myocardial level via different mechanisms. It is well known that in vivo opioids play a significant role in shock and stress; sometimes they act as positive and sometimes--as a negative factor; the mechanisms of these effects are still not clear. We have shown that the endogenous opioid antagonist tetrapeptide FMRFa (Phe-Met-Arg-Phe-NH2) acted as a preservative agent to the acute hypobaric hypoxia and hemorrhagic shock, blood pressure and respiration and also the time of life. The efficacy of FMRFa was higher than that of naloxone. It was suggested that one of the possible mechanism of FMRFa preservative action was the increasing of sympathetic nervous system activity mediated by the endogenous opioid system inhibition.