where abound and afree are pre-exponential factors associated with long ('rbound) and short (Z'free) lifetime, respectively. This ratio is identical to that measured by spectral decomposition  but the methods of time-resolved spectroscopy afford a higher sensitivity and precision of its determination. Increase in the negative potential of yeast cells brings about an increased intracellular accumulation of positively charged dye molecules. This leads to an increase in free dye concentration in the cytosol and, due to an equilibrium between the frce and the bound fraction, also a monotonic increase in the fluorescence intensity of the bound dye. Our first results show that, in addition to conventional methods using lipophilic cations such as TPP § the time-resolved fluorescence spectroscopy of diS-C3(3) could be used for precise monitoring of membrane potential changes in suspensions of yeast cells.