Changes in the mechanical performance of the right and left ventricles were studied before, during, and after abrupt occlusion of a major coronary artery, by directly recording from indwelling pressure cannulas and flowmeters. Analogue computers were used to derive additional pertinent variables, so that an engineering type of description of ventricular function was inscribed in as many as 22 variables with standard physical units. In 7 of the 9 animals in the series the anterior descending coronary artery was ligated at the time of operation, days or weeks before the experiment. The principal effects of acute occlusion of the left circumflex coronary artery in unanesthetized dogs were a reduction in peak ejection velocity, peak acceleration, and stroke volume of the left ventricle. These changes may be attributed to interference with the left ventricle as an impulse generator, since the pattern of ejection assumed some of the characteristics of the pattern of normal right ventricular outflow. Sustained tachycardia compensated for the reduction in the stroke volume, so that cardiac output was well maintained. The experiments in which large changes in left ventricular performance occurred were suddenly terminated by ventricular fibrillation within minutes or hours of the experimental coronary occlusion.