We examined cytoplasmic pH regulation in Schizosaccharomyces pombe and Saccharomyces cerevisiae using pH-sensitive fluorescent dyes. Of several different fluorescent compounds tested, carboxy-seminaphthorhodafluor-1 (C.SNARF-1) was the most effective. Leakage of C.SNARF-1 from S. pombe was much slower than leakage from C. cerevisiae. Using the pH-dependent fluorescence of C.SNARF-1 we showed that at an external pH of 7, mean resting internal pH was 7.0 for S. pombe and 6.6 for S. cerevisiae. We found that internal pH in S. pombe was maintained over a much narrower range in response to changes in external pH, especially at acidic pH. The addition of external glucose caused an intracellular alkalinization in both species, although the effect was much greater in S. cerevisiae than in S. pombe. The plasma membrane H(+)-ATPase inhibitor diethylstilbestrol reduced both the rate and extent of alkalinisation, with an IC50 of approximately 35 microM in both species. Amiloride also inhibited internal alkalinisation with IC50's of 745 microM for S. cerevisiae and 490 microM for S. pombe.