An iterative model is described to predict isothermal and nonisothermal elongation of an extruded filament at a given force. The principle of the model allows a quite simple description of Rheotens tests with much flexibility regarding the shape of the elongational viscosity function, including true viscoelastic flow and thermal boundary conditions. The algorithm is compact and computing time on a PC is short. The application of an integral K-BKZ constitutive equation to predict Rheotens curves of the well characterized LDPE Melt I is demonstrated and the influence of the slope of the transient viscosity in the strain-hardening regime is discussed. A simple approximation to take into account the prehistory of the material in the die based on purely elongational strains is proposed which includes the irreversibility assumption of the damping function (Wagner model). Predicted Rheotens curves are compared with experimental results.