In drilling technologies, Na-bentonite-water mixtures are used as drilling mud. As the reserves of sodium-bentonite decline, calcium or low-quality bentonites are activated with inorganic salts and/or treated with different types of polymers as an alternative to Na-bentonite. The quality of such bentonites, commonly referred to as extended bentonite may deteriorate under severe drilling conditions, such as / bacterial activity, high temperature etc. It was considered that borates could prevent the deterioration of extended bentonites due to their well known bactericide properties. This study involves the use of Na-Borate as an activating agent for bentonites in place of the commonly used Na-carbonate. The effects of Na-borate on the rheological and filtrate behaviours of two different bentonites, namely, Edirne Bentonite (E-Ben) and &Ccedil / ankiri Bentonite (C-Ben) were investigated. The results were evaluated based on API standards and in comparison to those of Na-carbonate activated bentonites. It was found that Na-borate could be used as a bentonite activator, although somewhat higher dosages (3,78% Na2O equivalent for E-Ben and 0,76% Na2O equivalent for C-Ben) than Na-carbonate (2,90% Na2O equivalent for E-Ben and 0,58% Na2O equivalent for C-Ben) are necessary to obtain the same rheological properties. In terms of aging, it was determined that aging had no adverse effect on v the rheological and filtrate properties of bentonite treated with carboxymethyl cellulose (CMC) and polyanionic cellulose (PAC), but rheological properties of bentonites treated with xanthan gum were found to deteriorate upon aging. The presence of Na-borate in the mud appears to alleviate this adverse effect.