An electromechanical servocontrol system was developed to analyze mechanical properties of a nonexcised, blood-perfused papillary muscle. A miniature load cell mounted on the moving arm of a pen motor measured muscle force, and a shaft angle sensor built in the motor measured the length. The pen motor was actuated by a servocontrol command signal. A papillary muscle was exposed in the right ventricle of an excised, cross-circulated canine heart. The tendinous end of the papillary muscle was connected to the load cell. The intact root of the muscle was fixed by a metal ring. The preparation was stable over 5 h heating at a regular spontaneous heart rate (95-150/min) at 37 degrees C. The system allowed the muscle to contract in various modes such as isometric, isotonic, auxotonic, afterloaded isotonic, quick released, lengthening, etc. Changes in the contraction mode, preload, and afterload could be achieved at the investigator's disposal simply by turning switches and potentiometers, or by feeding external command signals of desired wave forms.