A pedicled diaphragmatic skeletal muscle graft was used to replace a portion of resected right ventricle in 35 dogs. The graft contracted when electrically stimulated directly or via the phrenic nerve before and after insertion. The electrical pacing threshold was lower for phrenic nerve stimulation (0.9 +/- 0.20 mamp) than for direct graft stimulation (2.3 +/- 1.19 mamp). The heart could be captured and paced by stimulating the muscle graft with higher current (16.2 +/- 4.49 mamp). The delay from pacing stimulus to muscle graft contraction when the graft was paced directly was 10 msec. The epicardial activation time delay when the heart was paced through the muscle graft was 27.0 +/- 9.08 msec. When the muscle graft pedicle was transected, the graft lost its ability to contract. The heart, however, could still be captured electrically through the graft for up to 4 hours. Strain gauge studies of the nonstimulated muscle graft showed tension development during pre-ejection ventricular contraction identical to that of the right ventricle. In the ejection phase, muscle graft tension slowly declined. The stimulated muscle graft developed active tension and echographically demonstrated muscle thickening during contraction. This study demonstrates that a vascularized, neurally innervated diaphragmatic muscle graft can be placed into the right ventricle. The graft retains its ability to contract in response to direct or phrenic nerve stimulation. It can be made to contract during any phase of the cardiac cycle. Thus diaphragmatic muscle grafts may provide a method to augment ventricular cavity size with synchronously contracting muscle.