Degenerate four-wave mixing is analyzed in the case of a two-photon resonant transition between two levels with permanent dipole moments. A comparison is made with the single-photon transition case, where it is found that permanent dipole moments have no influence. Nevertheless, the difference between the permanent dipole moments of the two levels is relevant in the two-photon transition case; the reflectivity of the phase-conjugate wave increases as this difference increases.