Results are presented of temperature and pressure studies of the non-linear dielectric effect (NDE) in the isotropic phase of MBBA (N-4-methyoxybenzylidene-4′-n-butylaniline) and EBBA (N-4-ethoxybenzylidene-4′-n-butylaniline). In both nematogens the pretransitional increase of the NDE is described by the classical relation obtained on the basis of the Landau-de Gennes model. The value of the ratio of amplitudes describing the pretransitional effect of MBBA and EBBA shows a dependence on the measurement frequency of the NDE (a few MHz). In nematogens with a large positive anisotropy of the electric permittivity, change in the measurement frequency even leads to a change in sign of the NDE on approaching the clearing temperature.