In the present study the effects of heroin and morphine administration on cardiac function, myocardial enzyme activity and cardiac ultrastructure features have been evaluated in isolated and perfused rabbit hearts. In 24 male rabbits the hearts were excised and immediately perfused in a retrograde non recirculating Langendorff apparatus. Successively, heroin 1 x 10(-5) or 1 x 10(-4) M or morphine at 1 x 10(-4) M were individually administered in 18 rabbits for 45 min. In 6 rabbits, used as controls, the isolated hearts were perfused with perfusion fluid for the same time. At the end of perfusion period the hearts were removed and histology was studied. During the perfusion period, creatine phosphokinase (CPK), lactic dehydrogenase (LDH) activity, heart rate (HR), derivative of intraventricular pressure time ratio (dP/dt) and changes in systolic ventricular pressure (SVP), diastolic ventricular pressure (DVP) and in coronary blood flow (CBF) were also evaluated. At the end of perfusion period no significant changes in CPK, LDH, HR, dP/dt ratio, SVP, DVP and CBF in comparison with basal values were observed in all groups. Histological specimen were characterized by slight derangements in atrial and ventricular structure both after heroin and after morphine administration. In conclusion our data suggest that alterations following opioids administration might rather be related to a systemic action involving only indirectly the heart.