Changes in the electrical impedance of the thorax were recorded from various electrode arrays on the thorax during the infusion and withdrawal of saline, plasma, or blood from the right, left, or both hemithoraxes. Impedance changes correlated linearly with the volume of infused fluid. The resistivity of the infused fluid significantly affected the sensitivity (omega/ml) of the impedance method in detecting specific fluid accumulations. The use of hemithoracic electrode arrays permitted the localization of fluid accumulations to a specific hemithorax. Various factors that can alter the magnitude and interpretation of impedance changes are discussed, as they would affect the clinical interpretation of impedance changes, particularly in postoperative patients.