Saccharomyces cerevisiae and Schizosaccharomyces pombe cells were grown on D-glucose, D-galactose, D-fructose, D-mannose, maltose, trehalose and ethanol. All these substrates were separately added to cells thus grown and the onset and rate of acidification mediated by the plasma membrane H(+)-ATPase were determined. Irrespective of the growth substrate, the best triggers of acidification in both species were fructose, mannose and glucose (with average rates of 5.2, 5.0 and 4.8 nmol H+ per min per mg dry weight, respectively, for S. cerevisiae, and 4.5, 6.8 and 5.8 for S. pombe). These were followed in S. cerevisiae by galactose in Gal-, Man- and Tre-grown cells (about 0.40 nmol H+) and by maltose in Mal- and Tre-grown cells (about 0.15 nmol H+). Trehalose elicited some response in only ethanol-grown cells while ethanol itself was completely ineffective in activating the H(+)-ATPase. In S. pombe, however, maltose caused an acidification rate of 3.6 nmol H+ per min per mg dry wt., followed by EtOH (().38), Gal (0.13) and Tre (0.05). 6-Deoxy-D-glucose and 2-deoxy-D-glucose, not metabolized or improperly metabolized analogues of glucose, had no effect whatsoever. It appears that the sensor triggering the ATPase-activating pathway is a complex responding both to a glucose-type sugar (Glc, Man, Fru) and possibly identical with one of the glucose carriers, and to one of its metabolites, most probably fructose-6-phosphate.