This study was done to compare postnatal alterations in blood viscosity (capillary viscometer) and its determinants: hematocrit, plasma viscosity (capillary viscometer), red cell aggregation (Myrenne aggregometer) and red cell deformability (rheoscope) in the first five days of postnatal life in full-term neonates with early (< 10 s) and late (3 min) cord-clamping. The fetal blood volume of the placenta (“residual placental blood volume”) decreased from 52±8 ml/kg of neonatal body weight after early cord-clamping to 15±4 ml/kg after later cord-clamping. Neonatal blood volume, calculated as the difference between an assumed total feto-placental blood volume of 115 ml/kg and the measured fetal blood volume of the placenta, was 50% higher in the late cord-clamped infants than in the early cord-clamped infants. Both groups showed similar viscosity, hematocrit and other rheological parameters in cord blood. In the infants with early cord-clamping, the hematocrit decreased from 0.48±0.04 1/1 at birth to 0.43±0.61/1 after 24 h (p < 0.05). Whole blood viscosity did not change significantly with age. After late cord-clamping, the hematocrit rose from 0.50±0.04% at birth to 0.63±0.051/1 at 2 h of age (p < 0.005) and dropped to 0.59±0.51/1 (p < 0.05) at 24 h. Blood viscosity increased by 40% (p < 0.001) within the first 2 h, but did not change significantly during the following five days. In both groups, plasma viscosity and red cell aggregation increased significantly (p < 0.05) on day 5 due to significant increases in total plasma protein and fibrinogen concentrations (p < 0.01). Red cell deformation was not affected by the mode of cord-clamping and did not change with age. We conclude that late cord-clamping results in marked rise in blood viscosity due to increasing hematocrit. The postnatal rise in plasma viscosity and red cell aggregation may keep the blood viscosity at a high level despite falling hematocrit.