Two groups of seven patients with uncomplicated septal defects or pulmonary valve stenosis were perfused by the use of “Travenol” bag oxygenators and a heat‐exchanger for open heart surgery. A dilute perfusate was used, to which human serum albumin was added in one group for comparison with the other group. No clinical differences were found between the two groups, but hæmatocrit values suggested that the albumin prolonged the duration of hæmodilution into the postoperative period. The probable advantages of maintaining hæmodilution in this way are discussed.