Continuous tissue pH and intermittent central arterial pH were measured in six rabbits during 10-min exposures to a mixture of 10% CO2 and 90% O2. In control and recovery situations tissue pH was more acidic than arterial pH by a mean value of 0.07 pH units. During periods of rapidly increasing pCO2, the steady state relationship was inverted with tissue pH being more alkaline than arterial pH. After a second exposure to CO2, mean tissue pH values did not recover to baseline. It is concluded that in the rabbit during acute hypercarbia, the relationship of tissue to central pH is variable. The possible implications of these results in human fetuses during labor are discussed.