We studied the feasibility of augmentation of impaired myocardium with synchronously paced skeletal muscle grafts. The short contractile period of skeletal muscle required development of a new stimulator to ensure that the pedicle graft contraction would simulate that of the myocardium. In four dogs each, rectus and diaphragmatic muscles were wrapped around a balloon and electrically stimulated in synchrony with the electrocardiogram, varying stimulation currents and frequency of discharge during systole. For this purpose, a prototype hybrid stimulator was developed which senses the cardiac R wave and modulates the electrical output to the skeletal muscle by delivering a train of impulses of varying frequencies within the systolic intervals. The contraction characteristics in response to such stimulation were similar in rectus and diaphragmatic muscles, but the former developed higher maximum tensions because of the greater muscle bulk. Square-wave output was found to be more effective than sine-wave stimulation, and a single stimulating electrical pulse of 40 msec duration produced a maximum tension of 60 mm Hg lasting for 120 msec, whereas a train of 4 pulses within a 320 msec period was able to achieve a maximum tension of 100 mm Hg lasting nearly 400 msec, the latter approximating that of the myocardium. In six other dogs, the rectus muscle pedicle graft was used to replace a segment of the left ventricle (25.5% +/- 2.1% of left ventricular mass) excised under cardiopulmonary bypass. Left ventricular isometric contraction was studied using a left ventricular balloon with stimulators turned on and off. Significant augmentations of left ventricular maximum tension (+19.3% +/- 2.5%, p less than 0.001, paired t test) and left ventricular contractility (+38.3% +/- 9.4%, p less than 0.001) were achieved when the skeletal muscle grafts were stimulated. Thus, in this preliminary study, skeletal muscle graft properly oriented and stimulated is able to augment the left ventricular isometric contractile function after significant loss of left ventricular myocardial mass.