1. The rates of the Bohr shift of human red cells and some of its constituent reactions have been studied with a modified Hartridge-Roughton rapid reaction apparatus using an oxygen electrode to measure the progress of the reaction.2. The rate of the Bohr shift was compatible with the hypothesis that the transfer of H(+) across the membrane by means of CO(2) exchange and reaction with buffers is generally the rate-limiting step.(a) When the Bohr off-reaction was produced by a marked increase in P(CO2) around the cells, the half-time at 37 degrees C was 0.12 sec. In this case CO(2) was available initially to diffuse into the cells, the process being predominantly limited by the rate of intracellular CO(2) hydration.(b) When the Bohr off-shift was produced by an increase of [H(+)] outside the cell, P(CO2) being low and equal within and outside the cells, the half time became 0.31 sec. In this case, even at the start, the H(2)CO(3) formed by the almost instantaneous neutralization reaction of H(+) and HCO(3) (-) had to dehydrate to form CO(2) and this in turn had to diffuse into and react within the red cell before the [HbO(2)] could change. When a carbonic anhydrase inhibitor was added to slow the CO(2) reaction inside the cell, the half-time rose to 10 sec.(c) The Bohr off-shift in a haemolysed cell suspension produced by an increase in P(CO2) appeared to be limited by the rate at which the CO(2) could hydrate to form H(+).3. The Bohr off-shift has an average Q(10) of 2.5 between 42.5 and 28 degrees C with an activation energy of 8000 cal.4. The pronounced importance of the CO(2)-bicarbonate system for rapid intracellular pH changes is discussed in connexion with some physiological situations.