A pneumatic ventricular assist device (Sarns/3M) has been redesigned for low volume hydraulic actuation to accommodate muscle powered drive systems. Design modifications include adding a bellows/piston mechanism (to compress the blood sac) and a compliance chamber for volume compensation. A simple prototype device was constructed to measure the efficacy of piston pump actuation and to validate pusher plate design. Device manufacture was affected by removing the drive line housing from the pneumatic pump and replacing it with a piston/bushing mechanism. A convex piston profile was chosen to maximize ejection fraction and minimize device size. Stroke volume was found to be a linear function of piston displacement (∼3 ml/mm) and reached a maximum value of 45 ml. Mean compression forces of 46–56 N acting during a 12 mm stroke (2.1 L/min at 60 cycles/min) were sufficient to generate mean afterload pressures of 70–110 mm Hg in a mock circulatory loop. Peak compression forces ranged from 72 to 86 N and work input was calculated to be 552–672 ml/stroke. These data indicate that this method for delivering muscle power to the bloodstream is both mechanically viable and compatible with the functional capacity of conditioned latissimus dorsi muscle. ASAIO Journal 1999; 45:178–182.