A nonlinear integral equation describing the evolution of spheroidal deformation of a drop that is unstable with respect to its intrinsic charge is derived and solved for arbitrary values of viscosity. It was shown that, due to an essentially nonlinear character of the phenomenon, the characteristic time of instability development equals the time of tenfold increase in the amplitude of an initial, physically infinitesimal spheroidal deformation of an unstable drop. The dependence of the instability characteristic time on the drop viscosity is described by an increasing linear function. (C) 2000 MAIK "Nauka/Interperiodica".