The concept of toposelectively modified vesicles (toposomes) is presented. The application of an electric field vector constitutes a method by which these objects can be created. The effect of millisecond electric field pulses on giant (10-200 mu m) vesicles composed of either one of two polymerizable lipids was studied in the unpolymerized and partially polymerized states. It was found that the behavior of the vesicles depended strongly on the fluidity of the membrane. For N,N-dimethyl-N,N-bis[2-(tetradeca-2,4-trans,trans-dienoyl)oxyethyl]ammonium bromide (C-14, T-m = 20.5 degrees C) in the unpolymerized state, large (several microns) hole formation was observed in the direction of one of the electrodes followed by resealing. In the partially polymerized state, a similar puncturing was observed, but instead of resealing, the hole remained stable for 90 s. For N,N-dimethyl-N,N-bis[2-(octadeca-2,4-trans,trans-dienoyl)oxyethyl]ammonium bromide (C-18, T-m = 31 degrees C) in the partially polymerized state, an indentation, associated with either clefts or small pores, formed facing one of the electrodes. This morphological change was found to be stable for at least 20 min. Without polymerization the larger C-18 vesicles collapsed upon exposure to even submicrosecond pulses. However, for smaller (<20 mu m) unpolymerized C-18 vesicles, electroporation occurred apparently through stable invisible pores. Other electric-field-induced phenomena such as electrofusion and electrobirthing were also observed exclusively with the C-14 lipid. These results delineate the factors which will determine vesicle behavior upon exposure to an electric field vector and thus allow the rational choice of experimental conditions conducive to the fabrication of a particular type pore-bearing toposome.