This report concerns the feasibility of low volume priming extracorporeal circulation. Through this study, the bubble oxygenator with Zuhdi's heat exchange was used. Moderate hypothermia with surface cooling and hemodilution perfusion with 5 per cent D/W was evaluated in 32 mongrel dogs and 16 clinical open heart cases. The results obtained here were as follow: 1) Body temperature reduction by surface cooling before bypass provided more even cooling than did core cooling by low flow partial bypass alone. 2) In regard to cardiac loading on returning the whole perfusate of the circuit to patient, approximately 20 ml/kg of 5 per cent D/W was feasible as a priming solution. 3) To reduce the blood visicosity, hemodilution technique with 5 per cent D/W was superior, and hemodilution effect during postoperative periods was temporaly. 4) The excess lactate volume postulated by Huckabee was a available index to evaluate metabolic acidosis during the extracorporeal circulation. 5) With aid of surface cooling, the acid-base balance during perfusion was kept to lesser extent than that of core cooling only. 6) This study indicated that the low priming perfusion in conjunction with surface cooling hypothermia was a reliable technique for the open heart operation and may be applied in more prolonged perfusion.