The nonlinear responses of an entangled SBR melt and its solutions in rapid uniaxial extension have been studied by rheometric and rheo-optical measurements. During (startup) extension, the samples progressively lose entanglements. At relatively low rates, all entanglements are eventually lost, leading to yielding, nonuniform extension, and ductile specimen failure. At sufficiently high rates, some entanglements survive to allow non-Gaussian stretching and full chain extension leading to rupture-type sample breakdown when the failure mechanism switches from disentanglement, i.e., mutual chain sliding, to chain scission. The precursor to rupture, i.e., non-Gaussian stretching, is evidenced by the birefringence measurements that show the breakdown of the linear stress–optical relation. The corresponding critical stress is found to be proportional to the polymer concentration, in agreement with a scaling analysis based on an entanglement network picture. The onset of non-Gaussian stretching and rupture occurs at higher stretching ratios when the entanglement strands are made longer by dilution as achieved in the three solutions. In other words, both yielding and rupture occur at higher strains with increasing entanglement spacing, underscoring the legitimacy to represent entangled polymers in terms of an elastic network when depicting their predominant nonlinear responses to rapid uniaxial extension.