Six patients on maintenance hemodialysis were investigated by direct determination of hemodynamic changes induced by UF. Our results show: 1. Despite a significant fall in ECF and PV, the PV/IF ratio was unchanged indicating absence of major fluid shifts between intravascular and extravascular compartments. 2. The rise in CPV/TBV ratio observed indicates central redistribution of intravascular fluid suggesting venoconstriction. 3. CPV and hence CO was maintained so that a stable MAP resulted.