We investigate the con.nement properties of GaN/Al xGa1-xN zincblende and wurtzite nonabrupt quantum wells (QWs) in an electric field. It is shown that their Stark shifts decrease considerably when the existence of nonabrupt interfaces are considered. Confined excitons in 50 Å wide GaN/Al0.3Ga0.7N zincblende and wurtzite quantum wells (QWs) can exist up to electric field intensities of the order of 500 kV/cm. In all cases, the electric field effects in GaN/Al xGa1-xN wurtzite QWs are stronger than in similar zincblende QWs.