A theoretical and experimental study of the nonlinear properties of a waveguide, whose core is filled by a nematic L.C., is presented. Hybrid, homeotropic and planar alignments at the waveguide walls we considered in the strong anchoring approximation. In the case of the homeotropic alignment with negative optical torque a spatial splitting of the guided field is expected similar to the one already observed in a L.C. electro-optic device.