The best-known paper on non-Newtonian fluids in mixing systems is the Metzner and Otto's work. A central idea of their contribution is based on the assumption that in laminar flow there exists an average shear rate (<(gamma)over dot>(alpha upsilon)) around the impeller (whose exact location and geometric shape is not clearly specified), and that it is proportional to the impeller speed (N), i. e.,<(gamma)over dot>(alpha upsilon) =KsN, where K-s is the Metzner-Otto constant. In this work, an in-depth investigation of this assumption is carried out by using computational fluid dynamics (CFD) to calculate the three-dimensional flow induced by a Rushton turbine (RT). It was found that the volume swept by the blades is the region where K-s may be computed explicitly as K-s -<(gamma)over dot>(alpha upsilon)/N directly from non-Newtonian flow simulations. The obtained K-s values in this region were found in good agreement with reported data. Furthermore, power number measurements and data from literature were used to validate the simulations. The CFD method developed in this study can be used to readily and reliably evaluate Ks of industrial mixing impellers without resorting to power data of Newtonian fluids. (C) 2017 Elsevier Ltd. All rights reserved.