Abstract: Cardiac output is measured using an injected substance (indicator) that changes the conducting property (electrical resistivity) of blood, provided the indicator has a different resistivity from that of blood. A new type of electrically compensated tetrapolar conductivity cell located at the distal end of an intravascular catheter is disclosed. The conductivity cell consists of four electrodes arranged across the end of a catheter at its tip. The outer electrodes used for current injection are enlarged to reduce the electrode-electrolyte interface impedance. Capacitance compensation of the cather/tetrapolar conductivity cell provides reliable and repeatable blood resistivity measurements. The catheter further includes a calibration resistor thereby eliminating a calibration step previously necessary in using such a catheter for measuring cardiac output.