The process of acidification of the external medium after addition of glucose and subsequently of KCl to a suspension of yeast cells varies substantially from species to species. After glucose it is most pronounced in Saccharomyces cerevisiae and Schizosaccharomyces pombe but is very much lower in Lodderomyces elongisporus, Dipodascus magnusii and Rhodotorula gracilis. Both the buffering capacity and the varied effects of vanadate, suloctidil and erythrosin B indicate that the acidification is by about one-half due to the activity of plasma membrane H(+)-ATPase and by about one-half to the extrusion of acidic metabolites from cells. This is supported by the finding that a respiratory quotient greater than one (in various strains of S. cerevisiae and in S. pombe) is indicative of a greater buffering capacity and overall acidification of the medium. Taking into account the virtually negligible buffering capacity of the medium in the pH range where the effect of K+ is observed, the effect of K+ is generally of a similar magnitude as that of adding glucose. It is clearly dependent on (anaerobic) production of metabolic energy, quite distinct from the dependence of the H(+)-ATPase-caused acidification.