The flow properties of Egyptian bentonite as a drilling mud were studied using anionic polyacrylamide as an additive (MW 1,000,000 g/mol). The rheological properties (plastic viscosity, yield point, thixotropy) of bentonite-polyacrylamide mud were investigated at different concentrations (5-20 mg/L). It was found that, as polyacrylamide concentration increases, rheological properties increase after 24-h aging time and up to 40 degrees C. By increasing temperature, rheological properties become nearly constant. Electrical properties (electrophoretic mobility and zeta potential) of Na bentonite/PAM mud were measured using zeta meter. The results showed that zeta potential of Egyptian bentonite/PAM mud increased at low PAM concentrations up to 10 mg/L. At high polyacrylamide concentrations, these properties decreased. To investigate the relation between rheological and electrical properties, potential energy profiles were constructed. Potential energy profile at 10 mg/L polyacrylamide-clay showed a high repulsion potential energy between clay surfaces, i.e., suspension stability attained. The most reliable clay suspension to meet the desired properties could be reached using 6% clay suspension concentration, 10 mg/L polyacrylamide, and temperature 40 degrees C up to 24-h aging time. (c) 2007 Wiley Periodicals, Inc.