Voltage gating was investigated in transmembrane channel proteins! known as porins, extracted from E. coli strains 0111:B4 (mainly OmpF) and K12 (mainly OmpC). The reversible gating studied here was found to be one of several types of different channel closing phenomena observed when the porins were incorporated into an artificial bilayer membrane using the Montal-Mueller technique. Reversible gating occurred for transmembrane voltages in excess of approximately 100 mV and was found to be well described by a simple two-state switching model.