Skeletal muscle (SM) can be used for long-term circulatory assist. To generate contractions of appropriate duration and strength, SM requires bursts of electrical pulse trains. Presently, the implantable pulse train stimulator for SM assist is unable, at different heart rates, to adjust pulse burst duration or delay time, adversely affecting timing of counterpulsation. A prototype implantable microprocessor based stimulator (Medtronic Prometheus) has been developed to address this issue. The purpose of this study was to test this generator in an acute dog model. A dual chambered counterpulsation device was connected to the aorta in six dogs, using the latissimus dorsi to power the pump. The generator was connected by sensing/stimulating leads to the heart/left thoracodorsal nerve, programmed to give bursts of 2 V, 30 Hz, and pulse widths of 180 microsecs. Burst delay and duration was set as a fixed percent of the R-R interval of the EKG. The baseline aortic pressures ranged from 75/33 to 118/87 mmHg. During counterpulsation, diastolic pressure increased from 17 to 50 mmHg (30-111% increase). With changing heart rates, the pulse generator spontaneously readjusted its burst delay and duration, maintaining optimal diastolic counterpulsation without systolic interference. We conclude that this new pulse generator can stimulate SM for counterpulsation at varying heart rates, and is suitable for implantation in chronic studies.