The literature on the value and variability of the pH of the cytoplasm and vacuole of plant cells, and of the bathing medium, is reviewed. It is concluded that the pH of the cytoplasm and the vacuole changes much less than that of the medium during plant growth, despite a number of essential processes which produce or consume large quantities of protons in the cell during growth. It is thus concluded that loss of excess H+ or OH− to the medium is a major feature of cell pH regulation. In general, the excretion of excess H+ or OH− is an active process, i.e. against a gradient of electrochemical potential.The biochemical 'pH stat' mechanism of Davies is briefly discussed as an alternative to active transport of H+ or OH−. It is concluded that the major and primary function of active H+ and OH− fluxes is the regulation of cytoplasmic pH. Secondary functions (such as chemiosmotic ATP synthesis, transport of other solutes, and morphogenesis) of the H+ or OH− transport are also discussed. The extent to which these processes can occur is limited by interference with the regulation of intracellular pH.