Mitral flow patterns in acute myocardial ischemia of the canine heart were investigated by pulsed Doppler echocardiography in relation to the size of myocardial ischemia. Myocardial ischemia was accomplished and extended by ligation of the coronary arteries one by one. Effects of heart rate on the mitral flow patterns were also studied by atrial pacing. Results obtained as follows: 1. With increase of pacing rate, the flow velocity of atrial filling increased. Under constant pacing rate, myocardial ischemia caused reduction of the flow velocity of rapid filling, but it did not affect the flow velocity of atrial filling. 2. The flow velocity and acceleration rate of rapid filling decreased with extension of myocardial ischemia at pacing rate of 90 /min. The change of acceleration of rapid filling well correlated with prolongation of time constant of descending limb of left ventricular pressure. 3. These results indicated that analysis of mitral flow in acute myocardial ischemia was able to provide information about the existence and extent of extension of the ischemia.