Stress, strain, and optical birefringence were measured for cross-linked natural rubber in compression and tension over a range of strains. The experimental data were compared to the constrained-junction theory (Flory, P. J.; Erman, B. Macromolecules 1982, 25, 800) and to the constrained-chain theory (Kloczkowski, A.; Mark, J. E.; Erman, B. Macromolecules 1995, 28, 5089). These elasticity theories were found to fit both the stress and birefringence data quite well in tension. However, discrepancies arise when both compression and tension data are analyzed together. The differences between the models were minor, compared to their deviation from experiment.