The effects of bunazosin on the ischemic myocardium were investigated in isolated, perfused working rat hearts. Ischemia decreased the pressure-rate product and tissue adenosine triphosphate and creatine phosphate levels. Reperfusion did not restore the pressure-rate product nor the adenosine triphosphate levels completely. Bunazosin (5 x 10(-7) and 5 x 10(-6) mol/L) preserved the levels of adenosine triphosphate and creatine phosphate after 20 minutes of ischemia and increased the extent of recovery of the pressure-rate product during reperfusion. The results suggest that bunazosin protects the myocardium against ischemic damage.