1. 1.|Blood pH of an active amphibian (Bufo valliceps) and a dormant amphibian (Scaphiopus couchii) at different body temperatures was found to resemble the pH-temperature relationship for water. Their pH-temperature coefficients were -0.030 and -0.028 u/°C respectively. The blood pH of a heliothermic lizard (Dipsosaurus dorsalis) and a heterothermic mammal (Perognathus longimembris) had a lower temperature coefficient, -0.007 and -0.0085 u/°C respectively. Blood pCO2 of the amphibia, reptile and mammal decreased at low body temperatures, in conformance with the pH regime; the lizard and mammal were, however, acidotic relative to the amphibia, at low temperatures. 2. 2.|Data from the present study, and other studies, suggest that the manner in which vertebrates regulate in vivo pH at different body temperatures is quite variable, and that behavioural/physiological thermoregulators tend to have low temperature-pH coefficients. 3. 3.|The temperature-pH optimum for a tricarboxylic acid cycle enzyme, malate dehydrogenase, from P. longimembris) was about -0.008 u/°C, which corresponds closely for the temperature-pH coefficient for blood (-0.0085 u/°C). 4. 4.|The pattern of in vivo pH-temperature regulation observed in vertebrates would appear to be intimately correlated with the physico-chemical requirements for the maintenance of cellular metabolism. Neither the temperature-pH coefficient for in vivo pH regulation, or for optimal enzyme activity, necessarily parallel the observed temperature dependence of the pH of water. © 1978.