Summary.A method of blood volume determination based on the determination of the dilution of labelled corpuscles is described. Radioactive sodium phosphate is administered to rabbit B; after the lapse of some days, a known blood sample of this rabbit is injected into the vein of rabbit A. A few minutes later, the corpuscles of a known volume of the blood of rabbit A are secured, their phosphatide content extracted, and its activity measured. Corpuscles of a known blood volume of rabbit B are treated in the same way. From the ratio of the labelled phosphatide P content of the corpuscles of rabbit A and rabbit B, the total blood volume of rabbit A is calculated.An alternative and often preferable determination is based on the comparison of the activity of the acid‐soluble P secured from the corpuscle samples of rabbit A and rabbit B.The blood volume per kg of rabbit weight is found to be 42 c.c., per kg of chick weight 45 c.c.We wish to express our hearty thanks to Professor NielsBohrfor putting numerous facilities at our disposal, to Professor ErnestO. Lawrencefor the gift of the radioactive phosphate, and to Dr. H. Damfor his effective help in the experiments with chick