The behaviour of a cell in an electric alternating field has been investigated, taking into account the field-induced diffusion flows of ions through the membrane. We computed the difference in ion concentration between the internal and external sides of the membrane and the transmembrane diffusion potential induced by the external field. We also studied the effects of these flows on dielectric properties of a tissue in the radio frequency range. The value of the electric permittivity at low frequencies decreases gradually with the increase of membrane permeability for ions, while the electric permittivity at high frequencies is unchanged. These effects are especially important for analysis of the dielectric spectrum of a tissue or cell suspension which has undergone the influence of various physical or chemical agents, e.g. ionizing radiation or detergents.