ACCURATE measurements of the rate of regeneration of red cells after hzmorrhage, in which fallacies due to possible fluctuations in the volume of the blood were avoided, were first made by Boycott and Douglas? These experiments were made on adult rabbits, and the purpose of the present communication is to compare with these results the figures obtained with young rabbits and with old and young rats. I n comparing the rate of regeneration in different individuals it is necessary that the

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... lost by bleeding should in all experiments be approximately the same proportion of the total amount possessed by the animal. In comparing different species of animals it is also necessary that the comparison should be made between individuals at about the same stage of development in which the proportional rate of growth is not markedly dissimilar. Thus rats weighing 50 and 1 5 0 grim. both have 10.0 C.C. oxygen capacity per kilo. body weight: and rats of 75 and 1 2 5 grms. body weight will both add 50 grms. t o their body weight in about twenty-five days: With this 50 grms. of body they make 0.5 C.C. oxygeu capacity (0.37 (2) grms. hamoglobin, or about 28,000 million red cells). But in the case of the smaller rat this represents an addition of 0.5 to 0.75 c.c., or 2.7 per cent. per day; in the case of the larger animals, 0-5 to 1.25 C.C. or 1.6 per cent. per day. If, therefore, both animals were bled the same proportion of their total oxygen capacity, it would be expected that the smaller animal would regenerate the loss faster than the larger, since it is continually producing proportionally more red cells and hence has a relatively larger apparatus in working order.