Results are presented of studies of the nonlinear dielectric effect (NDE) for two frequencies (300 kHz and 6.1 MHz) of the weak measuring field in the isotropic phase of 4 heptyl 4’ cyanobiphenyl (7CB). For the smaller frequency the NDE increases markedly towards positive values on approaching the clearing point. For the higher frequency the Increase Is negative. Despite this difference both pretransitional effects exhibit similar, mean - field temperature character. The observed behaviour coincide with relations between the relaxation time of prenematic fluctuation and measurement frequencies.