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We have investigated the temporal evolution of the dielectric properties of Jurkat cells, a malignant human T-cell line, after exposure to microsecond and nanosecond pulsed electric fields. Suspensions with a 10% volume fraction of Jurkat cells were exposed to either 8 consecutive 1.1 kV/cm electroporation pulses of 100 μs, or 8 consecutive 20-kV/cm pulses of 300 ns, applied with a repetition rate of about 1 Hz. The dielectric spectrum of the cell suspensions before and after exposure was determined by means of time domain reflectometry dielectric spectroscopy. After correction for electrode polarization, the conductivity and permittivity of the cell membrane was derived by fitting the suspension spectrum to a combination of Maxwell-Wagner mixture model and single-shell cell model. Both exposure regimens lead to an increase in the conductivities of suspending medium and cell membrane indicating that membrane poration had occurred. This was confirmed by membrane integrity markers. Significantly higher membrane conductivities without uptake of membrane integrity marker suggest different pore formation mechanisms. Accordingly, dielectric measurements allow us to assess conformational and possibly induced functional changes of cells after exposure on biologically relevant time scales, and in turn, evaluate and compare the efficacy of chosen pulse parameters.