A method and apparatus for the instantaneous measurement of venticular volume using an intracavity electrical impedance catheter having plural pairs of spaced surface electrodes driven by a corresponding plurality of electrical signals, each of the signals exhibiting a different discrete frequency, and having plural pairs of spaced surface electrodes for sensing the potentials at predetermined locations within the ventricle. Switching means are provided for selectively coupling the drive signals to predetermined pairs of surface electrodes and for selecting the sensing electrode pairs to be utilized at any given time for read-back of the sensed potentials. The read-back signals are demodulated and converted from an analog signal to a digital quantity. Then, a digital computer is used to determine from the sensed digital quantities the extrapolated value of impedance corresponding to sources spaced an infinite distance apart. Knowing the extrapolated impedance value, the volume of blood in the ventricle can be computed using the formula V=.rho.L.sup.2 /R.sub.0 wherein R.sub.0 is the extrapolated impedance value, L is the distance between the spaced sensing electrodes spanning the ventricle and .rho. is the resistivity of the blood.