The elasticity of different natural rubbers and SBR rubbers has been studied in simple extension at constant strain rate. The true stress has been plotted as a function ofλ2−λ−1as suggested by the molecular theory. A series of straight lines which do not pass through the origin have been obtained, and consequently two parametersσ0andGhave been introduced. These results allow one to hypothesize that the tensile stress is the sum of two terms: a Gaussian entropic contribution and a further contribution reaching rapidly a steady state valueσ0.Theσ0term is found to depend only on the chemical nature of the rubber and may be ascribed to local interaction of the segments of the network, whereas the value ofGdepends on the degree of crosslinking and seems to represent the rubbery modulus of the kinetic theory. A number of experimental data for various natural rubbers and PDMS networks from the literature have been replotted in the same way in order to check the validity of our representation, and a satisfactory agreement has been obtained.