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The rate of trypan blue uptake of liver cancer cells, indicating cell death, when exposed to subnanosecond high electric field pulses, increased strongly when the temperature was raised above 37 °C. The exposure of Hepa 1-6 cells to 2000 pulses of 200 picosecond duration and electric field amplitudes exceeding 80 kV/cm induced cell death in almost 30% of the cells when the temperature was increased to 47 °C for the time of the pulsing. For temperatures at 37 °C and below, the same exposure to pulsed electric fields did not show any measurable effect. Even for the maximum elevated temperature of 47 °C, thermal effects were not found to cause fatalities for the time of exposure, which was, for 2000 pulses at a repetition rate of 7-9 pulses per second, on the order of 5 min. The effect of temperature on the electrical properties of the cell was measured by means of dielectric spectroscopy. The membrane voltages derived from these values were found to be too low to cause electroporation at room temperature. However, the reduced viscosity of the membrane with temperature is likely to reduce the threshold for poration, and together with the effect of multiple pulses, is considered to be the cause for the observed high death rate of the cells. This argument is supported by molecular dynamics simulations which show an increased probability for pore formation with temperature.