A model of ENH (Extreme Normovolemic Hemodilution) in the conscious swine was developed. A 4 level blood exchange was performed down to 6% hematocrit. In each level 40% of total blood volume was withdrawn and replaced with the same volume of hydroxyethyl starch. Below hemoglobin (Hb) 3g/dl and 2 hours after last exchange there was tissue hypoxia. 4 of 10 animals survived and only Hb after instrumentation had statistical difference with Hb <10.4 g/dl in non-surviving animals. During ENH there was tissue hypoxia, increase heart rate and cardiac output, divergent blood pressure, decrease oxygen delivery and systemic vascular resistance.