The effects of inorganic ions and of the hydrogen ion on oxygen-binding properties of most respiratory pigments are opposite. The addition of salt to the medium increases oxygen affinities, and the addition of H+ decreases oxygen affinities of crustacean hemocyanins. These oxygenation properties, as observed in vitro, suggest that the oxygen-transport system must adapt to ionic changes in the blood. In fact, decreases in the salt concentration of the blood of estaurine blue crabs are accompanied by increases in pH, probably resulting from the input of ammonia produced in deamination of the intracellular pool of free amino acids as the cells conform to osmotic changes in body fluids. The result is a stability of hemocyanin function until the blood becomes very dilute. As the acclimation salinity is reduced from 35 to 15 o/ooo, the ionic effects on respiratory transport are balanced and there is no change in total oxygen uptake. At 5 o/ooo salinity, however, the higher blood pH is manifested in an elevation of the total oxygen concentration of prebranchial blood, probably because the Bohr shift is no longer opposed by a critical level of salt in the blood. Under these conditions the role of hemocyanin in aerobic respiration is reduced at high environmental oxygen levels, but it may be enhanced in hypoxic uaters.