Small molecule permeabilization by multiple, high-field, nanosecond pulses. Fluorescence microscopic images of individual Jurkat cells in growth medium containing YO-PRO-1 (5.0 μM) or propidium iodide (7.5 μM) were captured immediately before and 5 min after exposure of the cells to 4 ns, 8 MV/m pulses and the fluorescence intensity change for each cell was measured by photometric integration. (a) Measurable influx of YO-PRO-1 occurs after 30 pulses delivered at 1 ms intervals (1 kHz repetition rate). At higher pulse counts, permeability of the cells to YO-PRO-1 increases and propidium iodide influx can be detected. (b) YO-PRO-1 influx is not detected when 30 pulses are delivered at 10 Hz and 100 Hz.