Abstract : The goal of this thesis was to improve measurement techniques of fluid volume variations in patients during hemodialysis. Extra and intra cellulars volumes were measured by bioimpedance spectroscopy and plasmatic volume change were determined from hematocrit measurements by optical or ultrasound methods. We have proposed a new and more accurate method for measuring hematocrit by ultrasound by taking into account the simultaneous concentration of hemoglobin and plasma protein concentration due to ultrafiltration. We have also tested a new method for measuring total body water from its resistance at high frequency which is more accurate in dialysis. We have also discussed artifacts due to position changes and fluid resistivity changes during dialysis as well as three methods for dry weight determination.