The in vivo blood oxygen binding properties of rainbow trout were studied in normoxia and hypoxia (PO2 55-60 mmHg). Owing to hypoxia the in vivo oxygen dissociation curve of blood was shifted to the left; the P50 value decreased from 37 mmHg in normoxia to 27 mmHg in hypoxia. Hypoxia also caused an increase in the erythrocytic volume, a decrease in the concentrations of ATP and hemoglobin inside the cell, and an increase in the intraerythrocytic pH. All these responses are interrelated. The swelling of the erythrocytes decreased the concentrations of ATP and Hb in the cell. This decrease changed the Donnan distribution of protons across the red cell membrane, thus increasing the intraerythrocytic pH. The increase in the intraerythrocytic pH was the main cause of the increase in the blood oxygen affinity. Owing to the increased blood oxygen affinity the oxygen transport from gills to tissues in hypoxia was only slightly decreased from that in normoxia. However, as the venous oxygen tension decreased in hypoxia, the diffusion of oxygen from tissue capillaries to the cells was probably slowed down in hypoxia when compared to normoxia.