Microwave irradiation (f = 36 GHz) changes the properties of distilled water within the first 1-10 min. The new state is retained for at least tens of minutes and manifests itself as changes in power density spectrum of periodic fading voltage fluctuations that are generated during discharge of a capacitor in which water is used as a dielectric. It is assumed that long-term changes in water properties mediate the effect of electromagnetic fields on biological systems.