1. It has been shown in vitro that serial measurements of blood water during ultrafiltration accurately reflect changing blood volume. 2. It has been shown that minute changes in blood hydration produce detectable changes in blood viscosity. 3. An ‘in-line’ capillary viscometer has been constructed which can be placed in parallel with an extracorporeal circuit to give a continuous semi-quantitative measure of blood viscosity during ultrafiltration or haemodialysis. By making serial measurements of blood water each ‘viscometer curve’ can be corrected to permit calculation of blood volume provided that the starting blood volume is known. 4. Blood volume changes of less than 1% can be detected in vitro and provided that blood volume changes solely as a result of the removal or influx of water it can be measured continuously to within an accuracy of 4% for volume changes up to 30% irrespective of starting packed cell volume or blood water.