The activity of a putative H+/K+ exchange system in the plasma membrane of yeast was studied following the alkalinization of the interior of vesicles prepared with lecithin and yeast plasma membrane containing pyranine entrapped inside. The fluorescence of pyranine was used as an indicator of the internal pH of the vesicles. The addition of monovalent cations produced an increase of the internal pH, probably due to the activity of an exchange system, allowing H+ to leave the vesicle in an exchange for the cation added. The system showed partial selectivity towards K+ against other monovalent cations, and it was inhibited by amiloride. The activity of this system required the presence of the yeast plasma membrane in the vesicles, and it did not produce important changes of the membrane potential of the vesicles. The exchange depended partially on the relative values of the internal and the external pH of the vesicles. The system shows low affinity for the cations, and appears to be different from the mitochondrial H+/K+ exchange system, which is non-selective toward the different monovalent cations. This system could be involved in the regulation of the internal pH of the cells when they accumulate high concentrations of K+.