Experimental results for the rheological behavior of bentonite-water mixtures and its modification when lignite is added in the mixtures, as a thinning agent, both at low and high temperatures are presented. These systems are used regularly for the formulation of drilling fluids for the drilling of oil and gas and geothermal wells. The experimental determination of the shear stress-shear rate relationships of the various slurry formulations with different bentonite concentrations are conducted with a variable speed rotational viscometer (0.01-600 rpm), at low and high temperature and after static aging of the slurries at 177°C for 16 hr. The stabilizing effect of lignite addition was described and plausible explanations of this behavior were studied. This is an abstract of a paper presented at the 7th World Congress of Chemical Engineering (Glasgow, Scotland 7/10-14/2005).