This Letter presents a proton fast field cycling Nuclear Magnetic Resonance study of the molecular dynamics in the region above the smectic A-isotropic phase transition in thermotropic liquid crystals. The proton spin-lattice relaxation time was measured as a function of temperature at different Larmor frequencies and the experimental results were interpreted in term of the Landau De Gennes theory. Mechanisms provided by both individual reorientational fluctuations and fluctuations of the local order parameter can be distinguished at the different Larmor frequency scales. (C) 2004 Elsevier B.V. All rights reserved.