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The temperature difference (Tg) across the wall of the left ventricle has been studied in the dog by percutaneous left ventricular puncture. The temperature is higher in the epicardial layer than in the endocardial layer. A difference ranging from 0.20 to 0.70 C was found in all experiments. The TG was found to be stable over a long period of time and was not altered by infusion of isoproterenol or norepinephrine despite significant hemodynamic changes. On the other hand, the TG was reduced markedly when acute anemia or hypoxia was induced. The results with hyperoxia were inconsistent, although the TG was increased in half of the dogs studied. The magnitude of the TG is probably due to the effects and counteractions of many factors. These may include uneven heat production in different layers of the heart, insulation of the pericardium and chest wall, and rate of heat removal by the coronary blood flow and by left ventricular cavitary blood flow. In these experiments the fall in TG associated with anemia and hypoxia is assumed to be due to increased heat removal by the disproportionately increased coronary blood flow. anemia; hypoxia; coronary blood flow; left ventricular heat production; left ventricular puncture Submitted on August 3, 1964