Transient elongational viscosities of polyethylene melts from converging flow (entrance pressure losses) and from uniaxial stretching after extrusion (Rheotens test) are compared with steady‐state viscosities obtained by isothermal homogeneous drawing in different elongational rheometers. The laboratory tests allow predictions on differences of the drawability (minimum film thickness) in tubular film blowing. If the shear viscosities of two low‐density polyethylene (LDPE) melts are the same, the material exhibiting the lower level of elongational viscosity can be stretched to higher total strains before break. The shape of the die is shown to have a significant influence on the subsequent stretching behavior due to preorientations imposed during extrusion. Converging flow decreases drawability, whereas diverging dies yield an increase of the maximum pulling speed before break. The temperature dependence of the drawdown force in the Rheotens test can be predicted from the flow‐activation energy of the melts and the slope of the elongational viscosity function versus strain rate. An approximation by the slope of the shear viscosity function versus shear rate is proposed. The variations of the Bagley correction and of the shear viscosity at a constant extrusion rate with temperature are interrelated to that of the shear viscosity measured for a constant shear stress. Time‐dependent normalized viscosities at constant strain rates of a PIB and a LDPE melt in planar elongation experiments are compared to the behavior in uniaxial and simple shear flow. There is evidence that uniaxial elongation represents the upper limit of strain‐hardening, whereas simple shear seems to characterize the lower limit of strain‐softening. This is an important consideration in choosing the most informative laboratory tests. In addition, new results concerning the influence of molecular weight distribution on elastic compliances and the shapes of the elongational viscosity functions are presented. A linear polystyrene(Mw=197,000)does not show a maximum of elongational viscosity if blended with another linear polystyrene ofMw=1,300,000.These results support the hypothesis that long chain branching (as in LDPE) is much more effective than molecular weight distribution in producing a pronounced maximum in the steady‐state elongational viscosity versus strain rate.