This study explores, as an alternative to sodium bentonite, the possibility of (Na, Ca)-bentonites to meet the required drilling mud properties. Activation of the bentonites was performed using the most popular (Na(2)CO(3)) and also the most controversial additive (MgO) and their blends. Upgrading the (Na, Ca)bentonites was elaborated on the basis of viscosity, swelling index, filtration loss and electrokinetic measurements. The combination of soda and MgO produced a significant synergy. (C) 2010 Elsevier B.V. All rights reserved.