Tensile creep tests, tensile relaxation tests and a tensile test with a constant strain rate are performed on injection-molded isotactic polypropylene at room temperature. A constitutive model is derived for the time-dependent behavior of semicrystalline polymers. A polymer is treated as an equivalent network of chains bridged by permanent junctions. The network is modeled as an ensemble of passive mesoregions (with affine junctions) and active mesodomains (where junctions slide with respect to their reference positions with various rates). The distribution of activation energies for sliding of junctions in active mesoregions is described by a random energy model. Adjustable parameters in the stress-strain relations are found by fitting the experimental data. It is demonstrated that the concentration of active mesodomains monotonically grows with strain, whereas the average activation energy for sliding of junctions and the standard deviation of energies suffer substantial down jumps in the vicinity of the yield point. With reference to the concept of dual population of lamellae, these changes in material parameters are attributed to transition from breakage of subsidiary (thin) lamellae in the subyield region to fragmentation of dominant (thick) lamellae in the postyield region of deformations. (C) 2003 The Society of Rheology.