The flow properties of Egyptian bentonite were studied using a combination of anionic polyacrylamide (MW ¼ 1,000,000 g mol
À1) and cationic polyethylene imine (MW ¼ 700,000 g mol À1 ). This combination gave a significant effect on the clay suspensions to be qualified in the field of drilling mud depending on the sequence their addition. It was found that, by adding 50 mg/L cationic polyethylene imine followed by 10 mg/L anionic polyacrylamide, the rheological properties were improved using 6% Egyptian bentonite suspension, especially at 208C. But, by reversing this addition sequence, the rheological properties of bentonite suspension were declined. The zeta-potential value of bentonite suspension in the presence of 50 mg/L polyethylene imine was À47 mV, while at 10 mg/L polyacrylamide was À55 mV. But, on addition of 10 mg/L polyacrylamide to bentonite suspension followed by 50 mg/L polyethylene imine induced a reduction of the zeta-potential value to À51 mV when compared with that of polyacrylamide alone. By reversing the addition sequence, a very high stable suspension having zeta-potential values of À82 mV was obtained. Potential energy profiles were constructed to investigate the relation between rheological and electrical properties. Potential energy profile at 50 mg/L polyethylene imine followed by 10 mg/L polyacrylamide to bentonite suspension produced a high repulsion potential energy between clay surfaces, i.e. the suspension stability improved. By reversing that sequence, a significant decrease in the energy barrier was observed. The most reliable clay suspension as a drilling mud could be obtained by using 6% clay suspension concentration, with 50 mg/L polyethylene imine followed by 10 mg/L polyacrylamide, at 208C up to 24 h aging time.