Almost all aspects of cardiac function are sensitive to modest changes of temperature. We have examined the thermal sensitivity of intracellular pH regulation in the heart. To do this we determined the temperature sensitivity of pHi, intracellular buffering capacity, and the activity of sarcolemmal acid-extrusion proteins, Na+-H+ exchange (NHE) and Na+-HCO3− co-transport (NBC) in guinea-pig isolated ventricular myocytes. pHi was recorded fluorimetrically with acetoxymethyl (AM)-loaded carboxy-SNARF-1 at either 27 or 37°C. At 27°C, intrinsic (non-CO2-dependent) buffering power (βi) was ˜60 % of that at 37°C. Acid-extrusion (Je) through NHE was ˜50 % slower than at 37°C, consistent with a Q10 of ˜2. In 5 % CO2/HCO3−-buffered conditions, in the presence of 30 μm cariporide to inhibit NHE, acid extrusion via NBC was also slowed at 27°C, suggestive of a comparable Q10. Resting pHi at 27°C was similar in Hepes- or 5 % CO2/HCO3−-buffered superfusates but, in both cases, was ˜0.1 pH units lower at 37°C. The higher the starting pHi, the larger was the thermally induced fall of pHi, consistent with a mathematical model where intrinsic buffers with a low principal pKa (e.g. close to 6.0) are less temperature-sensitive than those with a higher pKa. The high temperature sensitivity of pHi regulation in mammalian cardiac cells has implications for experimental work conducted at room temperature. It also has implications for the ability of intracellular acidosis to generate intracellular Na+ and Ca2+ overload, cardiac injury and arrhythmia in the heart.