Pulsed electric fields (PEFs) above a certain voltage threshold cause electroporation for microsecond pulses and intracellular effects for submicrosecond pulses. Models describing these effects often depend on the electrical properties of the cell, which are altered by the PEF. We used time domain dielectric spectroscopy to provide data for these models and to measure effects on cell suspension conductivity. We applied single 50-μs and 10-ns pulses to HL-60 cells, with the voltages chosen so the pulses have approximately the same energy. For 1.1-kV/cm, 50-μs pulses, the conductivity rose within a minute after the pulse and dropped dramatically approximately 40 min after the pulse. For 78-kV/cm, 10-ns pulses, we observed a brief delay prior to the conductivity rise and noted the same drop in conductivity after approximately 40 min. For both pulse durations, higher voltages frequently led to membrane poration followed by a gradual recovery approximately 30-40 min after the pulse. For 2-kV/cm, 50-μs pulses, we observed significantly more scatter in Trypan Blue uptake measurements due to stronger effects on the cell membrane. By using a cell model, we showed that a 50-μs pulse caused a much larger rise in membrane conductivity than a 10-ns pulse of the same energy.