The effect of prolonged administration of alcohol on mitochondrial function and high-energy phosphate (ATP) of heart muscle was investigated in dogs. Animals were divided into two groups, a control group and a group that received alcohol. In the experimental series, dogs received 400 ml of a 25% solution of alcohol added to the food and drinking water. Measurements were carried out after ethanol had been withheld for 2 days. Total myocardial blood flow, cardiac output, and myocardial O₂ consumption remained at control levels. Measurement of cardiac contractility using the maximal rate of left ventricular pressure rise (dP/dt_max) showed no change in animals exposed to alcohol. When the afterload of the heart was increased with angiotensin, a slight but not significant decline in cardiac contractility was observed. Activities of various intramitochondrial and extramitochondrial enzymes were measured in both groups. After alcohol administration, the primarily intramitochondrial isocitrate dehydrogenase diminished. ATP in heart muscle of dogs exposed to alcohol declined, and mitochondrial oxygen consumption and respiratory control indices diminished. These observations suggest that the primary lesion leading to alteration of myocardial performance is a biochemical malfunction of the mitochondria, which at this early stage is not reflected in changes in myocardial contractility.