The development of echo contrast agents that can provide reliable opacification of the myocardium after intravenous injection is an important advancement for the clinical application of contrast echocardiography. In this study, the hemodynamic effects and echocardiographic characteristics of a new lipid-fluorocarbon echo contrast agent, Aerosomes MRX 115 (ImaRx Pharmaceutical Corp., Tucson, Ariz.) were studied in six anesthetized ventilated pigs. Intravenous injection of this new agent in doses ranging from 0.0005 to 0.01 ml/kg produced significant measurable and visible myocardial opacification without any effect on heart rate, systemic pressure, partial pressure of oxygen, or left ventricular systolic function. The two largest doses (0.005 and 0.01 ml/kg), however, resulted in mild reversible increases in mean pulmonary artery pressure of 12 and 16 mm Hg, respectively. In four animals, epicardial images were obtained before and during coronary artery occlusion. Intravenous contrast injection during coronary occlusion permitted delineation of the hypoperfused myocardial segment. This capability may further expand the utility of contrast echocardiography.