The rheological behavior of polypropylene, PP, ethylene-propylene copolymer, EPR, and EPR/PP blends was studied. Zero-shear viscosity and elastic relaxation time were determined by least-squares fits by using a Carreau-Yasuda model with Arrhenius temperature dependency. The effect of PP and EPR molecular weight, ethylene ratio in EPR copolymer (E/EPR), and EPR concentration on the zero-shear viscosity and elasticity of EPR/PP blends was determined experimentally. Molecular weight effects are compared to theoretically expected relationships. EPR concentration effect and E/EPR ratio effects agree well with predictions made by using the Tsenoglou model.