An automatic control system has been developed to balance and control the output of an artificial heart. The system consisted of 2 linked negative feedback loops. The left ventricle was controlled by a Servo-Stroke Optimizer, which insured complete filling and full stroke operation of the left pump with each beat and changed the beat rate in accord with changes in aortic pressure. The right ventricle was controlled by a Servo-Variable Systolic Duration unit, which changed the stroke of the right ventricle to maintain the left atrial pressure within a preset band. In the initial animal studies, control pressures (aortic and left atrial pressure) were obtained from implanted transducers. More recently, a method has been devised whereby the control pressures were obtained from specific points on the left air line pressure wave, obviating the need for implanted transducers. The control system has been evaluated in a mock circulatory loop and in 9 calves with implanted artificial hearts. The system has provided balance of the 2 implanted ventricles and changes in flow rate in response to changes in peripheral resistance. The need for an operator to make manual adjustment to the power units has been considerably reduced. Further studies are indicated to evaluate the function of the control system in animals performing moderate-to-maximal exercise. Furthermore, the ultimate benefits of full stroke operation vs fill limited mode operation remain to be delineated.