Load-independent pressure-dimension analysis was applied in 13 open-chest, anesthetized dogs during either left atrial (n = 7) or right atrial (n = 6) infusion of prostaglandin E1. Right atrial infusion of prostaglandin E1 in doses from 31 to 500 ng/kg/min resulted in no change in any parameters studied, including mean arterial pressure, cardiac output, and systemic and pulmonary vascular resistances. Left atrial infusion of prostaglandin E1 produced dose-dependent reductions in mean arterial pressure and systemic vascular resistance but no change in the slope of the relationship of left ventricular stroke work to end-diastolic length, a load-independent index of ventricular performance. In contrast to findings obtained with load-dependent parameters, these results suggest that prostaglandin E1 has no positive inotropic effect in vivo.