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1. We have measured the rate of the exchange of O₂ between eel red blood cells and their suspending fluid in a modified Hartridge—Roughton continuous-flow rapid-reaction velocity apparatus using an oxygen electrode to follow the progress of the reaction.

2. The half-times for the uncomplicated oxygenation and deoxygenation reactions in red cells at 24° C were approximately 0·025-0·08 sec.

3. The rate of the Root shift in cell suspensions varied widely, depending on the initial condition of the CO₂-bicarbonate buffer system in the suspending fluid, with the rate of oxygenation or deoxygenation of the intracellular haemoglobin as an upper limit.

(a) The most rapid Root shift was produced by a change in extracellular P_CO2 with minimal contributions from CO₂ hydration—dehydration reactions in the suspending fluid or from ion exchanges across the membrane, and had a half-time as short as 0·040 sec.

(b) The slowest Root shift was produced by an increase in the extracellular lactic acid concentration in the absence of any form of CO₂ or in the presence of acetazolamide. This process is presumed limited by the rate of H⁺ or OH^- transfer across the membrane and had a half-time in excess of 10 sec.

(c) The Root off-shift produced by an increase in P_CO2 plus a decrease in extracellular pH showed no significant trend as temperature was lowered from 30° to 11° C.

(d) The Root on-shift produced by a decrease in P_CO2 and increase in extracellular pH had a half-time of 3 sec at 30° C, 9 sec at 24° C and 20 sec at 11° C. These changes appeared limited by the uncatalysed rate of extracellular CO₂ hydration.

4. Root off- and on-shifts in cell haemolysates at 24° C, produced predominantly by changing pH but with unavoidable subsequent readjustments of the CO₂-bicarbonate buffer systems, had an initial rapid phase with half-times as low as 0·01 sec. However, the curves were not monotonic, although they became so in the presence of carbonic anhydrase, indicating partial rate limitation by CO₂ reactions.