The deoxygenation rates of transformed erythrocytes were compared with those of normal discocytes by both stopped-flow and continuous-flow methods. Echinocytic and spherostomatocytic transformations were induced by various anionic and cationic drugs, respectively, without altering the oxygen affinity of haemoglobin, the cell volume or the membrane fluidity. The echinocytic transformation reduced the deoxygenation rate at slow-flow velocities (50 cm/sec), as detected by the continuous-flow method. However, at higher flow velocities (150 cm/sec) the rate was similar to that seen in normal discocytes. A close correlation between the degree of echinocytosis, the retardation of deoxygenation rate and the increase of suspension viscosity were observed. Microscopic observation of flowing erythrocytes revealed that the echinocytes scarcely deformed at the slower flow velocity, but clearly deformed at the higher flow velocity to various shapes resembling the flowing discocytes. Transformation to spherostomatocytes had no effect on the deoxygenation rate, which was comparable with that of the discocytes, and even the higher flow force did not induce any deformation. The retarded deoxygenation and the increased viscosity of echinocytes was probably due to an augmented stagnant layer around the cells (i.e. an increase of the hydrodynamic effective volume); this layer was reduced when the echinocytes were deformed with increasing flow force.