Two series of uniaxial tensile tests are performed on isotactic polypropylene with the strain rates ranging from 5 to 200 mm/min. In the first series, injection-molded specimens are used without thermal pre-treatment. In the other series of experiments, the samples are annealed for 51 h at 160 degreesC prior to testing. A constitutive model is developed for the viscoplastic behavior of isotactic polypropylene at finite strains. A semicrystalline polymer is treated as equivalent heterogeneous network of chains bridged by permanent junctions (physical cross-links and entanglements). The network is thought of as an ensemble of meso-regions connected with each other by links (lamellar blocks). In the sub-yield region of deformations, junctions between chains in meso-domains slide with respect to their reference positions (which reflects sliding of nodes in the amorphous phase and fine slip of lamellar blocks). Above the yield point, the sliding process is accompanied by displacements of mesodomains in the ensemble with respect to each other (which reflects coarse slip and fragmentation of lamellar blocks). To account for alignment of disintegrated lamellar blocks along the direction of maximal stresses (which is observed as strain-hardening of specimens in the post-yield regions of deformations) elastic moduli are assumed to depend on the principal invariants of the right Cauchy-Green tensor for the viscoplastic flow. Stress-strain relations for a semicrystalline polymer are derived by using the laws of thermodynamics. The constitutive equations are determined by five adjustable parameters that are found by matching observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. A noticeable difference is revealed between the mechanical responses of non-annealed and annealed specimens: (i) necking of samples not subjected to thermal treatment precedes coarse slip and fragmentation of lamellar blocks, whereas cold-drawing of annealed specimens up to a longitudinal strain of 80% does not induce spatial heterogeneity of their deformation; (ii) the elastic modulus increases with the strain rate for non-annealed specimens and decreases for annealed samples.