VERTEBRATE blood hæmoglobins discharge protons on oxygenation. This is part of the Bohr effect1. Recent work2,3 has shown that in hæmoglobins from a variety of terrestrial mammals ranging in size from mouse to elephant the smaller the animal the greater is the sensitivity of its hæmoglobin to changes in pH. Thus elephant hæmoglobin discharges only about 1.5 protons during oxygenation while mouse hæmoglobin discharges about 3.8. An approximate inverse linear relation exists between the number of protons discharged by the haemoglobin during oxygenation and the logarithm of the body-weight of the animal from which the haemoglobin comes : one additional proton discharged corresponds to a decrease in body-weight by a factor of about 300. The extrapolat on of this relationship to the weights of large whales suggested that whales might possess hæmoglobins with either a zero or small Bohr effect.