We have developed and evaluated a prototype external pulse train stimulator for use with a new technique of circulatory assistance. This involves the use of skeletal muscle (latissimus dorsi) wrapped around the thoracic aorta as an entirely autologous extra-aortic counterpulsation system. The stimulator has been successful in initiating both muscle contraction at the desired part of the cardiac cycle (diastole) and the necessary fibre-type transformation required for long-term work (fatigue resistance). The background to the development of the stimulator and the early haemodynamic results are described in this paper. The evaluation of the prototype design and the areas for future development are discussed.