Recombinant human erythropoietin [rhEPO (180 iu/micrograms); 1 or 10 micrograms polypeptide equivalent/kg] was intravenously administered daily to 6-week-old normal rats for 1 week. Rheologically, (1) blood viscosity at shear rate of 40-380 s-1 increased in a manner entirely dependent upon the haematocrit, (2) no change in erythrocyte deformability was detected by high shear rheoscopy, and (3) the velocity of rouleau formation in autologous plasma (at 7.5 s-1) decreased. Haematologically, rhEPO administration induced considerable polycythaemia with reticulocytosis in a dose-dependent manner, accompanying increased cell volume and decreased intracellular haemoglobin concentration, thus the density distribution of erythrocytes shifted towards low specific gravity. Plasma viscosity and plasma protein composition were unaffected by rhEPO-administration. In erythrocyte metabolism, no drastic alteration in the level of 2.3-DPG or ATP was detected.