The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematic region. A perturbation of the equilibrium by a static electric field of high strength (E-0 approximate to 10(7) V/m) is followed by a nonlinear dielectric relaxation process that, measured with the probing electric field E(omega) of small amplitude and variable high frequency (up to 100 MHz), reflects the rate of the domains formation. The fraction of the mesogenic molecules involved in the prenematic domains at different temperatures is estimated.