In acute experiments on Wistar rats, after replacement of 92-94% of the blood with solution of high-molecular modified hemoglobin (HMH), 10.1-11.2 g%, the latter's oxygen transport function was studied. Prior to and after blood replacement, the gaseous composition of the blood and alveolar air, acid-alkaline condition of the blood, curves of hemoglobin dissociation, hematological parameters and survival of animals, were studied. The HMH was shown to preserve the ability to uptake and release oxygen, after administration into the vascular bed. The HMH solutions transporting the oxygen, sustain the life in rats during 9.7 +/- 2.3 hrs at a lethal degree of hemodilution (hematocrit 3.9 +/- 0.7%). Nevertheless, owing to high affinity between the HMH and oxygen and to low oxygen capacity of the solutions, the HMH is unable to maintain the initial conditions for oxygen transport to tissues.