In a large series of canine heart-lung preparations, system volumes were determined by the T 1824 dye bolus teehnic of Hamilton. Results were compared with volumes independently determined by equilibration of label in a recirculating system. The mean differences of values in the largest group of experiments was 3.9 ml., and was not significantly different from zero. A small but significant mean difference in a second smaller group is discussed relative to the basic postulates of the method.