Abstract: Conductance catheter apparatus for measuring the volume of a biological chamber, the apparatus including a catheter employing a plurality of electrodes along its length, the plurality including outer transmitting electrodes and intermediate potential sensing electrodes between the outer electrodes, an alternating current source connected to provide alternating currents at high and low frequencies to the outer transmitting electrodes, processing circuitry connected to the intermediate potential sensing electrodes for processing potential signals to obtain conductance signals indicating high frequency and low frequency conductance and for using the conductance signals to determine volume in a manner that excludes parallel conductance through the wall of the chamber and surrounding tissue.