Summary 1. Systemic and coronary hemodynamics have been determined in a series of 10 intact anesthetized mongrel dogs by the Fick principle and nitrous oxide method respectively, before and during a constant intravenous infusion of hexadylamine (MRL 38, dicyclohexyl-piperidyl-ethylene hydrochloride). 2. Administration of hexadylamine produced peripheral and coronary vasodilatation, as well as increased cardiac output and coronary blood flow. 3. The coronary sinus oxygen content increased significantly, with narrowing of the myocardial arteriovenous oxygen difference. 4. Left ventricular work and myocardial oxygen consumption increased slightly but insignificantly.