Unilamellar bilayer vesicles of 193 nm diameter, prepared from the zwitterionic phospholipid dioleoylphosphatidylcholine (DOPC), are investigated by the use of the reversing-pulse electric birefringence (RPEB) method with the aim of elucidating the mechanism of vesicle polarization that results in transient induced birefringence of the solution. For this purpose. characteristic parameters of the observed course of the time-dependent birefringence, which ensues the sudden polarity reversal of the perturbation field, are examined as a function of the concentration of NaCl (ranging from zero to 5 x 10(-4) M) placed inside and/or outside the vesicles. Effects of varying the lipid concentration and the applied field strength are also reported. On the basis of the ionic strength dependence of the extent and rate of the temporary partial loss of birefringence upon field reversal, a slow ion polarization mechanism is proposed that is consistent with the experimental results.