We have developed a technique to improve the sensitivity of relative membrane potential measurements in mouse spinal cord cells using the fluorescent, anionic, voltage sensitive dye, DiBa-C4(3) (Oxonol) and flow cytometry. In order to attribute cellular fluorescence primarily to membrane potential, signal variability due to cell size and shape was reduced by dividing the log fluorescence signal from each cell by either its log forward angle light scatter or log side scatter signals. The use of these ratios in place of log oxonol fluorescence reduced the coefficient of variation of the distributions while leaving the changes in mean fluorescence largely unaffected. Kolmogorov-Smirnov analysis of pre- vs. postkainate stimulation (an excitatory amino acid) showed improved sensitivity of the assay with the use of this ratio technique.