At high external pH, the Chara membrane is known to switch to a new state in which the membrane potential is highly negative; it has been characterized as a passive diffusion potential for H+ (or 0H−). DCCD is shown to inhibit the increased conductance and the highly negative membrane potential associated with this state. DCCD also inhibits the plasmalemma H+-ATPase, as well as cytoplasmic streaming. The alkaline state of the membrane is shown to involve a decreased permeability to K+; this enhances the selectivity for H+ (or OH−) which results from the increased permeability to H+ (or OH−). Altering the cytoplasmic pH affects the membrane potential at both neutral and alkaline pH. It can also affect the ability of the cell to make the transition to the alkaline state.