We propose a novel approach for studying the reorientation process of nematic liquid crystals in the presence of a variable external magnetic field based on the fast field cycling nuclear magnetic resonance (NMR) technique. Thermotropic 4-cyano-4(')-octylalkylbiphenyl was aligned as a nematic monodomain with an external magnetic field that is suddenly turned off, inspection of the NMR line shape shows that the nematic monodomain disorders in nematic domains that have a continuous distribution of local orientations. The dynamics of the orientation from a mesoscopic-disordered system were determined by the application of a constant magnetic field. Both experiments were done as a function of temperature for the whole nematic phase and as a function of the intensity of the external magnetic field. The technique allows a clear identification of two regimes, a high frequency one where the magnetic torque governs the dynamics of reorientation and a low frequency one where thermal fluctuations are responsible for the dynamics. (C) 2003 American Institute of Physics.