We studied melt-theological properties of ternary blends of polypropylene and high-density polyethylene with an elastomer (natural rubber or ethylene-propylene-diene), at several blending ratios and shear rates. Measurements made on a cone-plate rheometer are presented and discussed as a function of blend composition. The effect of the shear rate on the viscosity and flow curve is discussed in terms of the exponent of the power low for a non-Newtonian liquid. The addition of an elastomer to the polyolefins blends changes the shape of the viscosity-composition curve; this change is discussed in terms of the possible morphology of the blend.