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The effects of nanosecond pulsed electric field (ns PEF) on the egg development of the medaka fish are investigated. A pulsed power modulator using a magnetic pulse compression circuit was employed to generate pulses of 0.5-20 kV pulses. Fertilized eggs of strain d-rR medaka were used. The ages of the experimental eggs were new laid, 24-h postfertilization, and 48-h postfertilization. Propidium iodide and fluorescein isothiocyanate (FITC)-dextran conjugate, and a fluorescence microscope were used to study the effects of ns PEF on the blastomeres and the extent of egg structure damage. The FITC was injected to experimental eggs by a microinjection system, which can deliver nanoliters of FITC without producing any damage to the egg structure. Each egg was set at the middle of a 2- or 4-mm cuvette, and a single pulse was applied. The control eggs were sham treated. After the pulse application, the eggs were observed under the fluorescence microscope until they hatched or died. By applying low electric field pulses (10 kV/cm), the effects were minimal and temporary. These eggs could recover and grow as fishes. By increasing the electric field (20 kV/cm), embryos showed abnormal growth with deformed body structure or missing organs. For higher electric fields (over 30 kV/cm), immediate extended damage were observed. The research was motivated by its application in regenerative medicine to control embryonic-stem-cell differentiation and proliferation.