Laplace equations were used for modeling of electrotonic potential in three-dimensional isotropic double-space RC medium. Solutions of Laplace equations for the case of rectangular current pulse stimulation using spherical electrode we obtain using Laplace transforms in imaginary space. Solutions in original space we got using numerical invert Laplace transform. It showed that the rising front of the transmembrane potential becomes less steep in regard to rising radius of the stimulating electrode and asymptotically reaches single-dimensional cable case (evenly distributed RC-circuit). The steady state value of transmembrane potential decreases with the increasing distance from stimulating electrode. It remains always positive when stimulus current is negative.