The purpose of this study was to determine the sensitivity of the thoracic electrical impedance change to blood-volume changes in various regions of the thorax. Using recently euthanatised dogs blood was infused in both a stepwise and a continuously varying manner into the ventricles, atria, lungs and isolated segments of the aorta. The results show that the impedance sensitivities to volume changes in the atria and aorta are similar, whereas the volume changes of the ventricles showed a significantly smaller effect. Equal blood-volume pulses into the lungs or aorta cause an impedance change of a similar magnitude, but estimates based on control-pressure changes indicate the effect of aorta volume change on the impedance to be less than 30%. The data, therefore, indicate that changes in volume of several of the intrathoracic cardiovascular structures are likely to be involved in the generation of a composite cardiogenic electrical impedance change.