The dynamics of electric field-induced pore formation (electroporation) is investigated in 186 nm diameter, unilamellar bilayer vesicles (Liposomes) prepared from the phospholipid surfactant dioleoylphosphatidylcholine(DOPC). Formation of reversible transient pores is detected via the spectral changes associated with the electron-transfer reaction between Fe(CN)(6)(4-) ions entrapped in the interior compartment of the vesicle and Ir(Cl)(6)(2-) ions placed in the hulk medium. Upon application of a high-voltage square pulse (E = 8 kV/cm) to the suspension, poration occurs only if the pulse length is greater than or equal to 200 mu s. The field-induced elongation of the time average spherical vesicles to prolate ellipsoid in the direction of the applied field E is detected through the transient birefringence of the system. The onset of elongation precedes the availability of pores large enough for Fe(CN)(6)(4-) to pass through, with an estimated radius of similar to 9.6 Angstrom. The average rate of radius growth of the reversible pores (4 x 10(-6) m s(-1)) is found to be 5 orders of magnitude smaller than that of irreversible electropores in planar black lipid membranes of similar phospholipids.