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At present the most satisfactory estimates of blood volume are obtained from summing simultaneous measurements ofplasma volume, made by a dye method, and cell volume made by a marked cell method (Reeve, 1948 METHODS GeneralThe best method of injecting exact quantities of solutions and withdrawing blood samples was found to be through a fine polythene catheter passed via an ear vein into the great veins near the heart. The technique of catheterization, which has many applications, is described in an appendix. The catheter was maintained patent by a very slow infusion of heparinized saline. Injected fluids were washed in with a little saline; the first portion of a blood sample containing the saline from the catheter was discarded.All animals were starved for 12-15 hr. before an experiment. A preliminary (blank) blood sample of 4 ml. was withdrawn. The suspension of marked cells and the T 1824 solution were then injected and washed in. About 8 min. later a 4 ml. blood sample was withdrawn, and after this at about * Work undertaken on behalf of the Medical Research Council.

l-Arginine labeled with carbon-14 at the guanido group was injected into rabbits, and the ratio of the rates of entry of radioactivity into the protein and urea was determined. This ratio, multiplied by the rate of arginine utilization in urea synthesis and by the ratio of protein to protein arginine weights, gives the rate of protein synthesis. The method applies to albumin and should apply to any plasma protein synthesized in those liver cells which also synthesize urea.

Rabbit plasma albumin was labelled with 118x, injected intravenously, and measurements were made of the radioactivity in plasma, urine, and feces over many days. In some experiments plasma radioactivity was fractionated into Ira-albumin activity and that of labelled breakdown products. Curves of these radioactivities were compared with corresponding curves predicted by four mathematical models. Each model included a vascular and extravaseular albumin compartment in transfer equilibrium, a radioactive breakdown products compartment, and an excretion compartment; but model A supposed I131-albumin catabolism to occur within the vascular system, model B within the extravascular compartment, model C within both, and model D within a separate compartment receiving albumin for catabolism from the plasma. The experimental data were reasonably well predicted by models A and C. However, model D, though data were insufficient for its complete validation, gave the best predictions and agrees with present knowledge of albumin catabolism.Various methods for calculating the rate of albumin breakdown are discussed. When calculations are based solely on the plasma radioactivity data, identical rates are predicted by models A, C, and D. When, as a valuable independent method, eatabolism is calculated from plasma and excreted radioactivities, an error (ordinarily small) is incurred unless account is taken of the rate of passage of Ira-albumin to the breakdown sites, and of the rate of excretion of the radioactive breakdown products.For the m e a s u r e m e n t of the rates of exchange of a l b u m i n between the plasma and tissue fluids, a n d the rate of b r e a k d o w n of plasma albumin, a first req u i r e m e n t is a satisfactory tracer-labelled albumin. A n u m b e r of labels such as C 1~ (1), N 1~ (2), S 85 (3), and 1 I~ (4), have been used, b u t there has been d o u b t w h e t h e r certain of these tracer-labelled preparations, a n d particularly those incorporating I ~31 (5), behave identically as does the animal's unlabelled