Respiratory minute ventilation (VE), breathing pattern, oxygen consumption (VO2) and arterial blood gases and pH were measured in freshwater turtles (Chrysemys picta) at 10, 20 and 30 degrees C while the animals breathed gases of varying CO2 concentration (FICO2 = 0, 2, 4, 6 and 8%). Increasing body temperature produced unequal increases in VE and VO2 such that VE/VO2 decreased. This relative hypoventilation led to a rise in PaCO2 and fall in pHa. Increasing FICO2 at all temperatures greatly elevated VE. The magnitude of this response increased with increasing temperature. Thus, paradoxically, there was an increase in both PaCO2 and CO2 sensitivity with increasing temperature. Increases in VE due to increases in temperature were primarily due to a shortening of the periods of breath holding. Although changes in VT contributed to changes in VE with increasing FICO2, the changes in f, due to shortening the periods of breath holding, contributed twice as much. In relative terms, increasing temperature had no effect on the CO2 response of any respiratory variable. Analysis of the data indicates that all changes which occurred in VE, PaCO2 and pHa with changes in body temperature can be explained by equal Q10 effects of roughly two on both metabolic rate and ventilatory sensitivity to changes in PaCO2.