Measurements of spin-lattice relaxation time and complex permittivity have been performed for monocrystalline and polycrystalline pyridinium Perchlorate as a function of hydrostatic pressure and temperature. The influence of pressure on the reorientation of the pyridinium cation was analysed. The potential shape and heights of the energy barriers for this reorientation in all three phases have been determined. For phase I and III the pressure dependence of the activation enthalpy was obtained.