The dynamics of electric field-induced transient pore formation (electroporation) is studied in 178 nm diameter synthetic unilamellar bilayer liposomes, prepared from the surfactant dioleoylphosphatidylcholine (DOPC). With Ag+ ions entrapped in the interior compartment of the vesicle and Br- ions placed in the bulk medium, details of the poration process are revealed through the pore-mediated production of (AgBr)(n) clusters which are detected spectrophotometrically in the UV. The field-induced elongation of the spherical bilayer shells to prolate ellipsoids is monitored through the transient birefringence of the system. Effects of the field strength E, pulse length, and number of the high-voltage square pulses applied are investigated, and threshold values of these parameters necessary for electroporation are established. It is found that polarization and elongation of the liposomes precede pore opening by 30 mu s, and that 75% of the entrapped Ag+ ions are ejected during a single pulse into the bulk solution, where the indicator reaction Ag+ + Br- --> AgBr takes place.