The oxygenation of hemoglobins is accompanied by the dissociation of protons. The number of protons discharged is inversely related to the size of the mammal from which the hemoglobin comes. The number of mercuric ions which are immediately bound by hemoglobins is approximately equal to the number of protons dissociated during oxygenation. Pretreatment of human hemoglobin by N-ethylmaleimide, which appears to bind only sulfhydryl groups prevents the binding of any mercuric ions under conditions when mercuric ions would otherwise be bound. These facts suggest that those mammals with higher metabolic rates will generally possess hemoglobins with a larger number of appropriately placed cysteine residues.