A triple aorto-coronary vein bypass was performed in a 56-year-old Jehovah's Witness. Neither donor blood, albumin nor artificial macromolecules were used. Ten litres of non-haemic fluid were given during surgery and an additional 4 litres during the first 18 postoperative hours. During perfusion the haematocrit fell to 19 vol% and total protein in serum to 29 g/l. Eighteen hours after surgery, the patient was extubated without any signs of oedema and with a positive water balance of only 2.2 l. Diuretics were not given. Acid-base and electrolyte values were within normal limits both during and after operation. This and similar cases reported in the literature show that during extracorporeal circulation it is possible to reduce the concentrations of red cells and plasma proteins to below 40% of pre-operative values without serious complications.