Optical second-harmonic generation (SHG) studies of a nematic liquid-crystal (NLC) cell have been performed using combined phase and intensity measurements upon variation of the magnitude and sign of an external electric dc field. A phase shift of 180 degrees in the harmonic field generated by the bulk NLC molecules was observed upon sign reversal of this field. We present here the first demonstration of SHG with electrically controllable phase shift. The contributions to the SH signal due to both the bulk NLC molecules and the background of the adjacent cell windows could he identified.