The sympathomimetic amine, mephentermine sulfate (wyamine) produces a striking elevation of the ventricular function curve in the isolated supported dog heart and in the dog with an open chest with a complete circulation. It appears to have little effect on total peripheral vascular resistance. Under controlled hemodynamic conditions, the administration of the drug was observed to increase myocardial O 2 consumption and decrease efficiency in the non-dilated heart. When filling pressure was high, however, the agent was observed to decrease the O 2 consumption and increase myocardial efficiency. These data have been interpreted as being consonant with the importance of Laplace's law in the relationship between the total tension developed by the myocardium and its O 2 utilization. The possible role of other hemodynamic parameters such as the duration of systole and the rate of development of ventricular pressure have been considered.