The elongational behavior of two polyethylenes with different structures, namely, a low‐density polyethylene and a linear low‐density polyethylene, has been investigated by isothermal melt spinning. The length along the spinline has been rescaled according to time. The evolution of the calculated elongational viscosity versus time has been compared to the transient elongational viscosities measured after imposition of a constant extension rate. The data demonstrate that these two situations, which are in many ways similar, give nearly the same results. Consequently, the viscosity which can be obtained from a fiber spinning experiment is a transient elongational viscosity. Differences in the elongational behavior between the long branched and the linear polyethylene are presented such as displayed in the spinning experiment. The strong influence on practical parameters such as melt strength or breaking stretch ratio is demonstrated.