The effects of intraoperative prebleeding and hemodilution with lactated Ringer's solution on hemodynamics, oxygen transport and lung water were studied in four patients undergoing extensive surgical procedures. The results were contrasted with those previously obtained from hemodilution of a group of patients with Plasmanate. The mean volumes bled were 1,950 milliliters in the lactated Ringer's solution group and 1,697 milliliters in the Plasmanate group. The posthemodilution hematocrit values were 26 per cent and 25 per cent, respectively. Cardiac output increased to only 115 per cent of the base line in the lactated Ringer's solution group but to 161 per cent in the Plasmanate group. Systemic oxygen transport was reduced to 80.0 per cent in the lactated Ringer's solution group, while in the Plasmanate group, it rose to 109.7 per cent. The lung water and alveolar arterial oxygen gradients were increased to 134 and 112 per cent in the lactated Ringer's solution group, whereas in the Plasmanate group, the lung water was reduced to 82 per cent, and the arterial alveolar oxygen gradient was reduced to 75 per cent. In these selected patients, hemodilution was well tolerated. Plasmanate hemodilution resulted in better compensatory changes than did hemodilution with lactated Ringer's solution in terms of oxygen transport and changes in lung water.