Lipid bilayers, normally a barrier to charged species and large molecules, are permeabilized by electric fields, a phenomenon exploited by cell biologists and geneticists for porating and transfecting cells and tissues. Recent molecular Simulation studies have advanced our understanding of electroporation, but the relative contributions of atomically local details (interface water and head-roup dipole and counterion configurations) and medium- and long-range electrostatic gradients and changes in membrane structural shifts to the initiating conditions and mechanisms of pore formation remain unclear. Molecular dynamics simulations of electroporation in several lipid systems presented here reveal the effects of lipid hydrocarbon tail length and composition on the magnitude of the field required for poration and on the location of the initial sites of field-driven water intrusion into the bilayer. Minimum porating external fields of 260 mV nm(-1), 280 mV nm(-1), 320 mV nm(-1), and 380 mV nm(-1) were found for 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC), 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC), and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) bilayers, respectively, and correlated most strongly with the bilayer thickness. These phospholipid systems share several common features including a wide, dynamic distribution of the headgroup dipole angle with the bilayer normal ranging from 0 to 155 degrees that is only slightly shifted in a porating electric field, and similar electric field-induced shifts in water dipole orientation, although the mean water dipole moment profile at the aqueous-- membrane interface is more sensitive to the electric field for DOPC than for the other phospholipids. The location of pore initiation, at the anode- or cathode-facing leaflet, varies with the composition of the bilayer and correlates with a change in the polarity of the localized electric field at the interface.