A combination of the Bersted model, giving the relaxation spectrum in terms of the molecular weight distribution (MWD), and the rubberlike-liquid model of I odge is used to describe the elongational viscosity for constant extensional strain rates in terms of the MWD. Predictions of this hybrid model are in reasonable agreement with experimental results for polystyrene, if one assumes a strain rate dependent truncation of the relaxation spectrum. The predicted effects of varying molecular weight and breadth of the molecular weight distribution on the extensional viscosity are presented. At constant weight average molecular weight and constant strain, a narrower MWD is predicted to yield an extensional viscosity-strain rate curve that is essentially shifted to higher strain rates relative to a broader MWD. Furthermore, at constant weight, average molecular weight, constant strain, and high strain rates, a narrower MWD is predicted to yield a higher extensional viscosity.