New rheological results with molten polypropylenes of different degrees of syndiotacticity are presented. The temperature dependence of their dynamic viscoelastic functions is correlated with the characteristic ratio. Relaxation time and Newtonian viscosity values are plotted as a function of molecular weight. The very high values of both rheological parameters found for the most syndiotactic poly-propylenes are explained considering the reptation model, which links conformational parameters with the viscoelastic response in the molten state.