In a prospective double-blind study of 40 male patients requiring aorto-coronary bypass surgery, a dose of 8 ng/kg/min of PGI2 was infused throughout extracorporeal circulation (ECC) (control group: glycine buffer). Between the two treatment groups there was no difference in preoperative hemodynamic parameters, number of preoperative infarctions, or number of implantated aorto-coronary vein grafts. Before, during and up to one day after ECC, extensive hemodynamic and hemostaseologic examinations were performed repeatedly (a total of twelve times in each patient). During ECC and due to inhibition of platelet function, platelet counts were significantly higher in the PGI2-treated group without clinical evidence for an increased bleeding tendency. Systemic blood pressure, heart rate and perfusion pressure did not differ significantly in the two treatment groups (with the exception of one control 60 minutes after onset of ECC). Peri- and postoperative function of the kidneys and lung also demonstrated similar results in both treatment groups. The platelet preserving effect of PGI2 correlated with a significant decrease of severe arrhythmias (tachyarrhythmias before termination of extracorporeal circulation) and a reduction in mechanical reperfusion. It is suggested that PGI2 exerts its beneficial effect on the myocardium by inhibiting platelet aggregation and/or by an antiarrhythmic effect on the myocardium. Whether the antiarrhythmic effect of PGI2 on the myocardium is indirect (reduction of microcirculatory disorders produced by inhibition of platelet aggregation) or direct (electrical stabilization of the myocardial cell membrane), is the subject of further investigations.