We study the effect of in situ synthesized 10 nm silica nanoparticles on the glass transition and dynamics of natural rubber networks using differential scanning calorimetry, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. Even in the absence of specific polymer-filler interactions, polymer segments within a few nanometers of the filler particles exhibit relaxation times up to 2-3 orders of magnitude slower and reduced heat capacity increment at the glass transition compared to bulk natural rubber. These effects are only observed when the nanoparticles are uniformly distributed in the polymer matrix. (C) 2011 Elsevier Ltd. All rights reserved.