Thirty-five indicator-dilution curves were recorded in dogs following the injection of indocyanine dye into the femoral vein, right atrium, pulmonary artery, or left ventricle. Sampling was from the femoral artery through a dichromatic cuvette densitometer. The exponential decay constant of each dye-dilution curve was calculated by a new method utilizing the phase plane and compared with the value obtained by the Stewart-Hamilton procedure. A high level of correlation was found between the two methods. Mean difference between areas was only 1.04 mg.1(-1).s (r = 0.999) and between mean transit times 0.02 s (r = 0.999). The phase plane method has the additional advantage of demonstrating the portion of exponential decay, enabling the elimination of indicator-dilution curves and abnormal downslopes.