The interaction forces between high molecular weight polystyrene brushes (M-n = (0.7-1.2) x 10(6) g/mol), formed by radical chain polymerization from an initiator chemically bound to silica (quartz-glass) substrates, were measured with a surface forces apparatus. Because of the high grafting densities obtained (7-10 nm between attachment points), the polymer chains were strongly stretched and extended several hundred nanometers from each surface in a goad solvent. The extension and magnitude of the purely repulsive force measured over the whole interaction range were compared to theoretical predictions. taking into account the polydispersity of the polymer. The differences in range and magnitude between forces measured on approach and on separation of the surfaces increased after long compression times and were attributed to entanglements within each compressed layer.