Oxygen dissociation curves were determined in mammalian blood at the CO2 tension of the organism, without the addition of buffers, dilution of the blood, or other alterations. It appears that the dissociation curve is related to body size in such a way that the blood of smaller animals has a higher unloading tension for oxygen. This finding is discussed in relation to the higher metabolic need for oxygen of the smaller animal. It is suggested that, in addition to the higher capillary density in the small animal, a higher unloading tension for oxygen also contributes to the steepness of the diffusion gradient for oxygen from the capillary to the tissue cells.