The significance of reductions in local myocardial flow on mechanical function and intramural electrocardiograms (ECG) was studied in anesthetized open-chest dogs. Local dimensional changes in the anterior wall of the left ventricle were recorded by ultrasonic technique. By platinum electrodes in the same region, both intramural ECG and hydrogen tension were recorded. Local flow was calculated from hydrogen desaturation curves. At approx. 25% flow reduction (constriction of the left anterior descending coronary artery (LAD)) local enddiastolic dilation and reduced systolic shortening appeared. Significant ST-segment elevation first appeared with flow reduction of 50%. After complete LAD-occlusions, enddiastolic dilation appeared within 20 sec, ST-segment elevation 40 sec later. It is concluded that mechanical and electrical events can be dissociated during acute myocardial ischemia: enddiastolic dilation and reduced systolic shortening are more sensitive indicators of moderate reductions in myocardial tissue flow than ST-segment elevation.