The cross-linking density at the filler-elastomer interface is analyzed by H-1 NMR measurements in model reinforced elastomers composed by grafted nanosilica particles and cross-linked ethylacrylate chains. We have focused our attention on the effect of introducing fillers on the relaxation of the bulk polymer matrix which is observed at long times (t > 100 mus). Measurements performed at high temperature (T > T-g + 120 K) have revealed that its relaxation is affected by the topological constraints existing at the particle surface. We deduce that the effect of particles in the bulk matrix can be interpreted as that of an homogeneous additional constraint density which increases proportionally to the surface area introduced in the matrix. (C) 2003 Elsevier Science Ltd. All rights reserved.