We studied the charging of cell membranes in response to ultrashort pulsed high electric fields with a temporal resolution on the same order as the electrical pulse, i.e. nanoseconds. The real-time resolution was achieved by using a pulsed laser (5 ns) as light source, together with a novel voltage-sensitive dye (Annine-6). The laser pulse was synchronized with the pulsed electric field to enable snapshots at different times before, during and after exposure. Electric fields were provided by a 50-Ω Blumlein pulse generator connected to a microreactor installed on a microscope. When Jurkat cells are exposed to a 60 ns, 100 kV/cm pulse, we observed changes in the transmembrane potential of up to 1.6 V at 15-20 ns after the electric field was applied. Within several tens of nanoseconds after the pulse, the transmembrane voltage returns to resting potential values, indicating that in spite of the extremely high transient electric fields in the membrane (3.2 MV/cm), the cell membrane is not permanently damaged.