The actions of intracoronary prostaglandins (PG) E1 A1, and F2α on coronary vascular resistance and cardiac function were studied in anaesthetized, open-chest dogs. PGEi produced potent, direct, coronary vasodilatation in both the beating, fibrillating and arrested heart. PG vasodilatation was not associated with increase in coronary A-V oxygen difference or myocardial oxygen consumption, nor with the release of K+ into coronary venous blood. PGA1was a less potent coronary dilator and PGF2α did not significantly affect coronary resistance or LV function. PGE1 and A1 both increased myocardial contractile force, reduced ventricular end-diastolic length, and in larger doses decreased systemic pressure. At constant heart rate an initial increase in cardiac output and venous return was followed by a more sustained fall.