The effects of Acetylcholine and Pilocarpine on coronary hemodynamics were studied using anesthetized open-chest dogs. The results were as follows: 1) Coronary vasodilating effect, decrease in heart rate, lowering in blood pressure and diminishing in myocardial contractility occurred immediately after the administration of Acetylcholine and Pilocarpine. (Muscarinic action) 2) Increases of blood pressure, coronary vascular resistance and myocardial contractility were recognized following muscarinic action (as mentioned above) by Pilocarpine, but not by Acetylcholine. The influence of an alpha adrenergic blocking agent (Phentolamine) on changes of blood pressure and coronary vascular resistance during 1 min. to 5 min. after the administration of Acetylcholine, but not of Pilocarpine, was observed. This suggested that the difference in reactions between Acetylcholine and Pilocarpine was probably caused by the different content of catecholamines released by the two drugs, and that not only alpha adrenergic receptor but also mechanisms other than the alpha adrenergic receptor were concerned in the recovery of blood pressure. 3) With Acetylcholine, coronary blood flow increased transientry. Decrease in coronary blood flow was observed with Pilocarpine. This was caused by increase in coronary vascular resistance and myocardial contractility. 4) After the administrations of these two drugs, myocardial blood flow to the outer-layer increased in accord with the increase of coronary blood flow, and myocardial blood flow to the inner-layer decreased in accord with the lowering of blood pressure.