The haemorheological disturbances observed in primary hypertension arise mainly from haemoconcentration, which leads to an elevation of blood and plasma viscosity and increased aggregation of red blood cells (RBCs). We evaluated the rheological properties of blood and central haemodynamic indices in 13 men with untreated primary hypertension (WHO stage I and II), during a baseline period and after intravenous infusion of 1000 ml of 0.9% NaCI (within 12-15 min). The rheological properties studied were: whole blood viscosity (WBV), plasma viscosity (PV), haematocrit (HCT) and plasma fibrinogen concentration (PF). The central haemodynamic indices were: mean intra-arterial blood pressure (MAP), central venous pressure (CVP), cardiac index (Cl), stroke volume index (SVI), total peripheral resistance index (TPRI) and the vascular hindrance index (VHI). Plasma renin activity (PRA) and plasma noradrenaline concentration (P-NA) were also measured.Volume expansion with saline caused haemodilution as expressed by a fall in HCT (P<0.001), WBV (P< 0.001) and PV (P<0.01). At the same time, CVP, MAP and VHI increased (P<0.05) while PRA decreased (P<0.05) and P-NA remained unchanged. Mean values of the cardiac index (Cl) and stroke volume index (SVI) did not change significantly. We did not observe any significant relationship between haemodynamic and haemorheological parameters, during baseline or between their respective changes after the infusion.The results indicate that although hypervolaemic haemodilution produced by saline infusion in hypertensive patients may improve blood flow properties (HCT, WBV, PV), blood pressure (BP) is not reduced; rather the converse is true. The reduction in HCT and hence the improved blood rheology, did not affect calculated vascular resistance. Thus, correction of WBV does not acutely normalize BP in primary hypertension.