Dielectric relaxation studies have been performed on trans -4- n -octyl-(4-cyanophenyl)is cyclohexane (8PCH) at ambient and high pressure (0.1-175MPa). Two experimental set-ups were applied: a time domain spectrometer (TDS) covering the frequency range 10 MHz-5 GHz, was used to study the relaxation processes in the isotropic phase (at ambient pressure); an impedance analyser (1 kHz-13MHz) was used for high pressure measurements on both the nematic (N) and isotropic (I) phases. The low frequency (l.f.) relaxation process connected with molecular rotations about the short axis is hindered by the activation enthalpy of 70 kJ mol-1 and 32.6kJ mol-1 in the N and I phases, respectively, whereas the high frequency process (rotations about the long axis) has an activation enthalpy of 22.6kJ mol-1 (isotropic phase). From the pressure and temperature dependencies of the l.f. relaxation time tau, the activation volume, enthalpy and energy were calculated. It was found that the energy barrier hindering the molecular rotations around the short axis in the nematic phase is influenced to about one half by the volume effects. The nematic potential q was estimated at various pressures and comprises 10 20% of the total energy barrier. The pressure dependence of q enabled the calculation of the order parameter S (p) with the aid of old (Maier and Saupe) as well as recent (Coffey et al.) theoretical formulae.