The effect of an alpha adrenergic blocker bunazosin on cardiac compensatory reactions was studied on Wistar rat myocardial infarction model. Heart weight/body weight ratio was increased and left ventricular peak dp/dt, the first time derivative of the ventricular pressure, was decreased in untreated myocardial infarction rats. Myosin heavy chain V3 component, plasma atrial natriuretic peptide (ANP) and atrial and ventricular ANP mRNA were increased in these animals. Bunazosin treatment suppressed all these changes while systolic blood pressure was lowered following the treatment. Hydralazine, a vasodilator with direct effect on vascular smooth muscles, equally lowered systolic blood pressure. However, decrease in left ventricular peak dp/dt, changes in the myosin heavy chain isozyme pattern and elevation of plasma ANP and cardiac ANP mRNA were not affected by hydralazine. The present observations suggest that the beneficial effect of alpha adrenergic blocker bunazosin on decompensated heart may result not only from cardiac unloading but also from direct alpha blocking effect on myocardial cells.