The effect of low dose prostacyclin (PGI2) infusion (0.025 to 0.035 g/kg per min) on regional blood flow distribution in acutely ischemic left ventricular myocardium was studied in nine open-chest anesthetized dogs. Blood flow was measured using left atrial injection of "15" micron radioactive microspheres 30 minutes following ligation of one to four branches of the left anterior descending coronary artery and again 7 to 9 minutes later during continuous PGI2 infusion when mean aortic pressure was reduced by 11 mmHg (P < 0.005). Heart rate, cardiac output, left atrial and pulmonary artery pressures and coronary sinus PO2 were unchanged during PGI2 administration. PGI2 had no effect on transmural flow in either ischemic (0.32 ± 0.05 (SE) ml/min per g) or non-ischemic (0.82 ± 0.06 ml/min per g) myocardium. The regional distribution of blood flow in ischemic and non-ischemic myocardium was also unchanged during PGI2 administration. Specifically, ischemic tissue subendocardial flow (0.21 ± 0.05 ml/min per g) was not increased (0.20 ± 0.06 ml/min per g). We conclude that low dose PGI2 infusion reduces systemic pressure but has no effect on either the transumral or regional distribution of blood flow in acutely ischemic canine myocardium.