A study of the voltage induction of transient pores in phospholipid bilayer vesicles is reported. Unilamellar vesicles (dipalmitoylphosphatidylcholine), with a size distribution of 100 +/- 30 nm, were prepared by the method of Enoch & Strittmatter [Enoch, H., & Strittmatter, P. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 145]. The vesicles loaded with [14C]sucrose and suspended in a mixture of 150 mM NaCl and 272 mM sucrose (both are the isotonic solvent for erythrocytes) were exposed to an intense electric field in the range of 20--40 kV/cm, with a field decay time of 5--15 micro second. A transient leakage of sucrose label was detected when the field strength exceeded 30 kV/cm. After the field was removed, no slow leakage of the tracer molecules occurred during a 65-h incubation period at the room temperature (23 +/- 2 degrees C). The leakage is attributed to the field-induced transmembrane potential, but not other effects such as the Joule heating or the shock wave associated with the voltage discharge. When the potential exceeded a threshold value of 200 mV, corresponding to an applied field strength of 30 kV/cm, there was a dielectric breakdown of the bilayer structure. Pores which allowed passage of sucrose were formed, transiently. Experiments show that these pores were fully reversible, and no global and permanent damages to the vesicle bilayer were detected. The implication of this membrane potential triggered conducting state of lipid bilayers to biological functions of cells is discussed.