The transient uniaxial elongational viscosity of BASF Lupolen 1840D and 3020D melts has been measured on a filament stretch rheometer up to Hencky strains of 6-7. The elongational viscosity of both melts was measured at 130 degrees C within a broad range of elongational rates. At high elongation rates, an overshoot or maximum in the transient elongational viscosity followed by a steady viscosity was observed. The steady elongation viscosity was about 40%-50% less than the maximum at high strain rates. The steady elongational viscosity as a function of the elongation rate,, decreases approximately as epsilon(-0.6) in both melts at high strain rates. The transient elongational viscosity, measured at a specific elongation rate at 170 degrees C on the BASF Lupolen 3020D melt, did not follow the time temperature superposition principle based on linear viscoelasticity during the decrease in the transient elongational viscosity towards the steady state. (c) 2005 The Society of Rheology.