The effect of nanoscopic, shape persistent polystyrene (PS) nanoparticles on the rheological properties of linear PS is studied and a dramatic viscosity reduction is observed. This is an ideal blend which simplifies enthalpic interactions between the components and can be used to delineate the effect of particle size on the properties of the blends. Homogeneous blends are assured through small-angle neutron scattering (SANS) experiments which establish the absence of phase segregation (depletion flocculation) in the nanoparticle-polymer blend. We previously found that nanoparticles reduce the viscosity of high molecular mass linear PS when the interparticle gap is smaller than the linear polymer size. In the present study, we find that such confinement of entangled polymers is necessary for the viscosity reduction since lower concentrations provide a viscosity increase. Furthermore, the behavior is found to be dependent on the presence or absence of entanglements and confinement is seemingly not important for unentangled polymers. It is proposed that constraint release caused by the addition of nanoparticles is responsible for some of the observed changes in viscosity although it is suspected this is a very complicated phenomenon and introduction of free volume by the nanoparticles is certain to play a key role.