Erythroleukemia K562 cells and lentil (Lens culinaris) root protoplasts have been subjected to pore-forming electric fields suitable for transfection experiments. Evidence is presented that the amount of hydroperoxides formed in cell membranes of both cell-types is a function of field strength applied. On the other hand, the electroporation-induced lipid peroxidation paralleled the enhancement of membrane permeability and was associated with greater membrane fluidity. The membrane hydroperoxides formed upon electric shock enhanced cell luminescence, and lipoxygenase activity appeared to be involved in the process. Electroporation of prokaryotic cells of Escherichia coli also enhanced light emission, which was higher in lipoxygenase-expressing clones.