We have examined the effects of morphine in auricular myocardium from non-failing and failing human hearts. In both preparations morphine induced inhibition. These responses were not antagonized by naloxone. Comparison of mean IC30 values obtained in non-failing (3.9 (SEM 0.2) x 10(-8) mol litre-1) and failing (5010 (200) x 10(-8) mol litre-1) hearts indicated that the morphine concentration-response curves were significantly (P < 0.001) shifted to the right in preparations from failing hearts. In addition, a decrease in the maximal response was observed. These data indicate that opioid receptors are not involved in the cardiac effects induced by morphine and that there is a decrease in responses to morphine in the failing heart.