The physical gelation of the solutions of atactic polystyrene (aPS) in carbon disulfide (CS2) is studied from the excess of anisotropic light scattering, deltaH(v). An abrupt jump of deltaH(v) at gelation temperature shows that a part of the chains is stiffened upon gelation. This effect which is independent of aPS molecular weight strongly decreases with concentration. The compatibility of this abrupt jump with the thermodynamic theories used to interpret the concentration and molecular weight dependence of the gelation temperature is discussed.