To analyse the limit of the stimation of stroke volume by thoracic impedance plethysmography (SVz), we considered an elastic tube forced with a trapezoidal flow input as a model of the aorta, and, based on Kubicek's equation, the volume input (SVa) was related to its impedance change via the model system parameters such as elasticity, fiuid inertia, peripheral resistance, total impedance across the tube and the rise and fall time of the input. SVz is equal to SVa only when the inflow is a square wave. The ratio SVz/SVa decreases with increasing input rise time, while it increases with increasing fall time, if the maximum flow rate and ejection time of the inflow are held constant. SVz hardly changes in association with changes in elasticity, fluid inertia, peripheral resistance or total impedance. A part of the results, the relationship between SVz, transthoracic total impedance and aortic flow waveform, was also demonstrated in dogs.