Four series of tensile relaxation tests are performed on isotactic polypropylene at elongations up to the necking point. In the first series of experiments, injection-molded samples are used without thermal pre-treatment. In the other series, the specimens are annealed for 24 h prior to testing at 110, 120 and 130 degreesC, respectively. Results of mechanical experiments are compared with DSC measurements. A constitutive model is derived for the time-dependent response of semicrystalline polymers at finite strains. A polymer is treated as an equivalent temporary network of macromolecules bridged by junctions (physical cross-links, entanglements and crystalline lamellae). At random times chains separate from their junctions and merge with new ones (the viscoelastic response), whereas junctions slip with respect to their positions in the bulk material (the viscoplastic behavior). The network is thought of as an ensemble of active meso-regions with various potential energies for detachment of chains from temporary nodes and passive meso-domains, where separation of chains is prevented by surrounding radial and tangential lamellae. Experimental data demonstrate that the content of active meso-domains increases with elongation ratio driven by the release of constrained amorphous phase induced by fragmentation of lamellae. In the sub-critical region of deformation (relatively small strains), the growth of the concentration of active meso-regions is associated with breakage of subsidiary (thin) lamellae developed at annealing. In the post-critical region (large strains), an increase in the fraction of active amorphous domains is attributed to disintegration of primary (thick) lamellae.