The nature of pH changes with temperature is similar in both air-breathing and water-breathing poikilotherms having a pH-temperature slope of -0.015 to 0.020 U/degrees C. The means by which this is accomplished are quite different in the two groups. The air-breather behaves in vivo as a Rosenthal, or constant CO2 content system, with PaCO2 increasing at higher temperatures. The increase in PaCO2 is brought about by a reduction in the ration VE/MCO2. The water-breather, on the other hand, does not change VG/MO2, so as not to sacrifice oxygen delivery, and consequently does not control PaCO2 by ventilation and perhaps does not control it at all. Instead, the ratio of CT/SCO2 is controlled by altering blood bicarbonate concentration. Since alteraction of Na+ and Cl- uptake rates affects blood pH, and alterations of blood pH affect uptake of Na+ and Cl- at the gills, the carrier-mediated Na+/H+(NH4+) and Cl-/HCO3-(OH-) mechanisms now known to occur in gills appear to be the primary pH-regulating mechanism in water-breathers. Our knowledge of the role of gut and kidney in pH regulation is incomplete, but on the basis of preliminary information the kidney does not appear to respond to acid challenge.