Under conditions of experiment, effect of outer hypoxia on the hemoglobin heterogeneous system and respiratory blood characteristics were studied in the golden mullet (Liza aurata R). The control fish groups was maintained at oxygen pressure of 158-162 gPa, the experimental group at 54-55 gPa. Exposure--15 days, water temperature--15 degrees C. Under conditions of oxygen deficit in the mullet body there is noted development of complex of compensatory reactions directed to maintenance of the oxygen blood capacity. Initially (the 1st-2nd day), the release of erythrocytes from spleen is noted, which increases the number of red blood cells and the hemoglobin concentration in blood (emergency adaptation). Then (the 10th-15th day), quantitative readjustments occur at the level of the hemoglobin system, which lead to an increase of blood affinity to oxygen and to a decrease of its sensitivity to pH (long-term adaptation). The revealed changes take place on the background of a decrease of volume of circulating erythrocytes, which reflects the character of change of erythrocytic (MCV, MCHC) and seem to be a consequence of partial dehydration of blood plasma.