The frequency dependence of the nonlinear dielectric increment was studied for benzene solutions of 4.4'-n-hexylcyanobiphenyl (6CB), 4-( trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) and 1-[3-n-decylbiphenyl]-2-[4-isothiocyanatophenyl]-ethane (10TPEB) molecules. In dilute solutions of these strongly dipolar nematogenic molecules (c < 1M) we observed the Langevin saturation related to nonlinear reorientation of dipoles in a strong electric field. The spectrum is described by the relaxation functions formulated by Coffey et al. [Proc. R. Ir. Acad. 78, 17 (1978)]. For more concentrated solutions (c > 1M) of 6CB and 6CHBT in benzene large positive field-induced changes of permittivity are observed. Such a large nonlinearity in polarization is expected in the vicinity of the isotropic-nematic phase transition because the molecular correlation length of the nematic domains becomes unusually large.