Abstract: Coronary haemodynamics was investigated by the coronary sinus catheterization and N₂O desaturation method in 10 dogs. Another 10 dogs were used for cardiac haemodynamic studies. Each dog received i.v. twice reserpine (0.25 mg/kg) and once the solvent(ascorbic acid), at intervals of at least 1 week. Effects on myocardial consumptions of O₂, glucose, lactic acid, pyruvic acid, amino acids, and esterified and nonesterified fatty acids were determined 30 min and 24hr after the i.v. reserpine. Ascorbic acid produced no change in coronary circulation. Reserpine brought forth a hypotension in 30 min. The arterial pressure decreased about 36% after 24 hr, accompanied with marked bradycardia and bradypnea. The pressure in coronary sinus was simultaneously lowered. Coronary blood flow was increased at 30 min, and then decreased at 24 hr. Coronary vascular resistance was reduced throughout the experiment. Reserpine produced no significant alteration in cardiac haemodynamics. Under normal conditions, myocardium utilizes mainly fatty acids and carbohydrates as its energy source.Thirty minutes following i.v. reserpine, the blood glucose level elevated, and the myocardial consumptions of O₂ and lactic acid increased, indicating a stimulation in aerobic metabolism. After 24hr, the consumption of O₂ was reduced while the lactate/pyruvate ratio was elevated, indicating that the anaerobic metabolism was enhanced. To epitomize, reserpine causes biphasic changes in coronary circulation and myocardial metabolism. Their possible relation to the release and depletion of catecholamines in myocardium have been discussed.