The measurement of left ventricular volume by the conductance-catheter technique has many advantages, but it is difficult to determine absolute volumes with this method. Current procedure requires that a bolus of concentrated hypertonic saline be injected to measure absolute volume. It also demands that the subject be in a steady state and that measurements only be made at discrete intervals. The saline bolus may affect the cardiovascular state of the subject. This paper describes a new technique for estimating absolute volume utilizing the conductance catheter that relies on the different frequency responses of blood and muscle. Good correlation between the salt-injection method and the dual-frequency method was found in nine-closed-chest pigs anesthetized with pentobarbital sodium (r = 0.922). Further refinements may extend the utility of the dual-frequency approach.