In the course of developing a technique for heart and lung transplantation a 6 kg Rhesus monkey model was used. We required a low prime circuit, because it was difficult to obtain donor blood for these animals, which was also capable of supporting the circulation for up to two hours. We describe a system incorporating a semidisposable bubble oxygenator, run under controlled negative pressure with very short blood lines. This arrangement allows for a priming volume of 110 ml with good venous drainage control. The principle could be exploited for use with paediatric extracorporeal systems.