The U.S. Bureau of Mines has continued its investigation of chemical additive drilling fluids that produce a zero surface charge (zsc) on a rock to better define the boundary conditons of the phenomenon. Laboratory drilling tests were performed on Minnesota taconite, Tennessee marble, and Sioux quartzite using diamond-impregnated coring bits or tungsten carbide spade bits. Drilling fluids were made from chemical additives such as inorganic salts, cationic surfactants, cationic polymers, acids, or nonionic polymers in either distilled, deionized water, mine pond water, mine well water, or tap water. These additives were tested below, at, and above their respective zsc concentrations. Penetration and bit life improvements obtained with zsc concentration solutions ranged from 88 and 56 pct, respectively, to over 650 pct and over 400 pct, respectively, for the additives tested. Polyethylene oxide (peo) was found to be the best additive for zsc-controlled drilling because a continuous range of peo solution concentrations produced the enhanced zsc-controlled drilling performance.