Among 17 species of Amazon fishes, the respiratory properties of the blood are clearly correlated with the gas exchange medium. A comparison between species shows that bimodal and obligatory air breathers have, in general, a higher blood oxygen carrying capacity, higher levels of intracellular modulators of oxygen binding, lower blood oxygen affinity, and a larger Bohr shift than water breathers. Within a facultative air-breathing species (Synbranchus marmoratus) removal from water and acute exposure to air (13–44 h) resulted in similar changes in blood respiratory properties. The significance of the respiratory adaptations of the blood, however, is clear only when physiological variables such as blood pH and [Formula: see text] are considered.