The effects of hydralazine and prostaglandin E1 on regional myocardial function were studied in dogs. Sixteen dogs were randomly assigned to one of two drug treatment groups of eight dogs each. The first group (G1) was treated with 0.4 mg/kg hydralazine administered as a bolus. The second group (G2) received prostaglandin E1 given as an infusion for a total dose of 0.8 micrograms/kg. Regional myocardial function was assessed through the measurement of myocardial segment shortening during systole. We call this index percent systolic shortening (%SS). An ischemic heart preparation was created by partial occlusion of coronary blood flow. The degree of induced ischemia was determined by following the reduction in %SS. Hydralazine reduced %SS of the ischemic myocardium while increasing the cardiac index, stroke volume index, and coronary blood flow. Prostaglandin E1 increased %SS, cardiac index, and stroke volume index in the ischemic heart preparation. Hydralazine, therefore, induced dissociation between global ventricular function and regional myocardial function whereas prostaglandin E1 did not. The present findings emphasize that evaluation of vasoactive drugs should consider their effects on regional myocardial function as well as on global hemodynamics.