Effect of anti-anginal drugs on the distribution of blood flow in the left ventricular free wall has been studied using thirty anesthetized open-chest dogs. The regional myocardial blood flows in the inner-(subendocardial) and outer-(sub-epicardial) layer were continuously monitored by heated cross-thermocouples designed according to Grayson's heat exchange principle. The techniques used were essentially the same as that described by Uchida (1970). The antianginal drug (nitroglycerin, 20 μg/Kg; prenylamine, 1 mg/Kg; dipyridamole, 0.3 mg/Kg; or dilazep, 0.2 mg/Kg) was administered intravenously. The results were as follows: (1) No difference in the regional myocardial blood flow between the inner- and outer-layer was observed. The flow ratio of the inner- to the outer-layer (I/O ratio) was 0.97±0.17. (2) Nitroglycerin produced a slight decrease in the flow through the inner-layer in spite of a remarkable drop in coronary perfusion pressure, while it did a marked decrease through the outer-layer. As the result, I/O ratio was significantly elevated after the injection. (3) Prenylamine produced a moderate increase in the flow through the both layers in spite of a remarkable fall in the coronary perfusion pressure. The ratio was rather elevated after a significant initial decline. (4) Dipyridamole caused a marked increase in the flow through the both layers, whereas I/O ratio was only minimaly lowered. (5) A change similar to that caused by dipyridamole was observed except a slight elevation of the ratio from 20 to 30 minutes when dilazep was injected. As mentioned above, these anti-anginal drugs produced different effects on the flow in the both layers. One of the explanation of these differences observed might be that these drugs have different effects on sites of coronary vessels, large intramural conductive vessels or small resistive vessels. Another explanation could be that these drugs cause different effects on the systemic hemodynamics. For the studies on effect of anti-anginal drugs on the intramyocardial distribution of blood flow, it is necessary that the regional myocardial blood flow must be monitored continuously since the intramyocardial distribution of the blood flow can be changed from time to time after the administration.