A new method was developed for in situ visualization of changes related to electropermeabilization of plant tissue. Onion epidermis stained with neutral red was subjected to a pulsed electric field (PEF), and serial images were captured by a camera connected to a stercomicroscope. Sample resistance was recorded simultaneously. Above a threshold level of a field strength of 0.35 kV/cm, it was possible to distinguish the individual permeabilized cells by their colour. Over 90-95% of the PEF-induced colour changes occurred during the first 2-3 min after the electric pulse and the rest after some 20 min. The size and rate of the observed changes were correlated with the severity of PEF, and were further influenced by the pH and conductivity of the solution used in mounting the epidermis, the sampling location on the onion epidermis, and cell size and number. The final conductivity increase was directly proportional to the number of permeabilized cells. Permeabilization was not randomly distributed but occurred along preferential paths connecting the electrodes. (C) 2002 Elsevier Science Ltd. All rights reserved.