Abstract Observations are reported on a thermoplastic elastomer (ethylene–octene copolymer) in uniaxial tensile tests. The experimental data reveal a rather unusual mechanical response: unlike particle-reinforced rubbers, for which preconditioning causes a monotonic decrease in tensile stress compared with that in a virgin specimen, cyclic preloading of the thermoplastic elastomer induces a reduction in stress at small strains and its noticeable growth at relatively large elongation ratios. A constitutive model is derived for the elastoplastic behavior of a polymer network with constrained chains at three-dimensional deformations with finite strains. The stress–strain relations involve five adjustable parameters that are found by fitting the experimental data. Good agreement is demonstrated between the observations and the results of numerical simulation. It is shown that the material parameters are affected by intensity of preloading in a physically plausible way.