Candida albicans blastospores harvested from 8 h (exponential) or 48 h (stationary) cultures were incubated with increasing doses of amphotericin B (AmB). The time course of H + influx and K + efflux was monitored by in vivo 3 1 P NMR and K + atomic absorption respectively. AmB was shown to be more active on exponential phase cells than on stationary phase cells. For both growth phases, K + leakage ocurred before pH acidification. In light of these results, together with iodoacetate experiments, it seems difficult to assert that K + leakage is a secondary effect resulting from an increase in the permeability to protons, as formerly proposed. In addition, no H + over K + selectivity of pores formed by AmB could be detected. Finally, some unexpected results were afforded by 3 1 P NMR experiments: a broadening of P i signals was detected on exponential phase cell spectra when the blastospores were incubated with 10 - 3 and 10 - 4 M AmB reflecting a transient heterogeneity of the intracellular pH within the cell population. For stationary phase blastospores, two subpopulations (IIa and IIb) were detected; population IIb, with a more acidic pH i , was much more sensitive to AmB action.