The loss of 18O from labeled CO2 caused by the exchange of oxygen with water, a process catalyzed by carbonic anhydrase, has been measured in suspensions of rat erythrocytes at pH 7.4 and 25 percent C. The rate of loss of 18O from all CO2 and the rate of loss of 18O from doubly-labeled CO2 are shown to be related to the rate constant for the catalyzed hydration of CO2 inside the cell and a rate constant for the diffusion of CO2 out of the cell. The results show that the diffusion of CO2 out of the cell with a half-time near 2 msec is a slower process than the intracellular, catalytic conversion of CO2 to HCO3- which has a half-time near 0.3 msec. From this information we estimate the gradient of 18O content in CO2 in the red cell during an 18O-exchange experiment. The rate constant for the entry of CO2 into red cells, also obtained from 18O-exchange data, has a value of the same magnitude as that anticipated for the diffusion-controlled rate of encounter between CO2 and red cells. This indication of diffusion-controlled depletion of 18O from Co2 is supported by experiments with a carbonic anhydrase inhibitor which show that carbonic anhydrase does not have a rate-limiting role in the 18O exchange until greater than 80% of the enzyme is inhibited.