Acid-base regulation is one of the most tightly regulated physiological processes among vertebrates due to the effect of pH on protein charge and thus function, affecting cell ion transport, muscle contractility, metabolism and ultimately survival. During short-term (up to 96h) exposure to elevated CO2 (hypercarbia), most fish studied to date exhibit pH compensation of the blood which is associated with a net increase in plasma HCO3– in exchange for Cl–, predominantly through processes at the gills. While deep sea fishes are thought to be very sensitive to acid-base disturbances, a concern given proposed strategies for CO2 sequestration, the deep residing Pacific hagfish is extremely CO2 tolerant and has an unparalleled capacity for net plasma HCO3–/Cl– exchange which likely represents the ancestral state for fishes. Due to an apparent limit to net HCO3–/Cl– exchange, most fishes examined to date cannot pH compensate at CO2 tensions greater than 10-16 mm Hg; in CO2 sensitive fishes, this may be the basis for mortality. A few fish species, however, are capable of tolerating PaCO2s well above 10-16 mm Hg; in some of these species, this tolerance appears to be associated with the ability to completely regulate intracellular pH (preferential pHi regulation) of tissues, such as brain, muscle and liver, despite a large reduction in blood pH. I hypothesize that preferential pHi regulation in fish evolved in the ancestors of the basal freshwater (non-teleost) actinopterygiians, is associated with high CO2 tolerance, and may have been an exaptation for air-breathing and the transition of life from water to land. (Funded by NSERC Canada).