The quantitative relations of the volume of the circulation (C), in cc. per min., to respiratory metabolism (R), the number of cc. of O consumed or CO2 exhaled per min., and the difference (D) in the amount of O and CO2 in arterial and venous blood respectively are defined as R/D = C. Thus if R=240 cc. O consumed, an average basal metabolism, and D =8 volumes %, then C = 3000 (3 liters per min.). If D =6 volumes %, the circulation is 4 liters; if D =4, the circulation is 6 liters. In other words, the volume of the circulation for any given metabolism and the arterio-venous difference vary inversely. This relation was first recognized by Fick; but the credit for the method of determining the volume of the circulation in man by means of the respiratory gases is, the authors say, erroneously assigned to Fick, since blood can not be taken from the right heart in a man. It was Hal-dane, Douglas and Christiansen who first devised a method of obtaining from the air of the lungs data equivalent to those from blood. This Haldane method can, however, be accurate only when confined to determinations of CO2 in the presence of sufficient O to fully oxygenate the blood. When used in this form the method affords determinations of the circulation which, during rest and on experienced subjects, are sufficiently reliable to serve as a standard against which the ethyl iodide method may be tested. Results by the 2 methods are found to be in satisfactory agreement. Both methods indicate an arterio-venous difference of about 3.5 volumes % CO2. This figure corresponds to an arterio-venous O difference of about 4 volumes % and indicates therefore a circulation of 6 liters per min. when 240 cc. of O per min. are consumed.