Presented experimental results bring an insight into the long-standing problem of the primary reinforcing mechanism in rubbery polymers filled with fine fillers. In this project the effect of silica nanoparticles on the linear viscoelastic response of model polyacrylamide (PM) hydrogel systems was examined. Using water as a dispersion medium allowed for separating the effect of filler-filler and the filler-polymer interaction. Rheological data suggest that the primary mechanism of the nano-reinforcement in filled PM hydrogels comes primarily from confinement to the PM chains and bias of their conformational states in respect to their reference states. In hydrogels with very low concentration of PAA, the transition between the filler-filler and filler-induced conformational restriction mechanism of reinforcement was captured. Published by Elsevier Ltd.