The Kerr constant of two monotropic nematagens with different molecules length is measured. The low temperature of the nematic-isotropic liquid phase transition which is typical for monotropic LC created the conditions for the observation of two Kerr-effect dispersions at the frequency lower than 10 MHz. It was shown that the law frequency dispersion process is connected with cooperational orientational molecules motion whereas high frequency is connected with the rotation of the individual molecules of nematogen.