In pursuit of innovative methods to improve mining productivity, the Bureau of Mines is investigating the use of inorganic salts as drilling fluid additives for penetration enhancement. Laboratory diamond drilling tests were performed on Westerly granite with aluminum chloride solutions and on Sioux quartzite with aluminum, aluminum nitrate, calcium, sodium, and zirconium chloride solutions as drilling fluid additives. These drilling results were compared with the average drilling performance using distilled, deionized water on Sioux quartzite (12 replicate tests) and Westerly granite (3 replicate tests). Maximum increases in penetration per given time and bit life were obtained when drilling with zero point of charge (zpc) concentration solutions of each additive. For Sioux quartzite, maximum increases in penetration and bit life ranged from 96 to 115 pct and 64 to 99 pct, respectively. For Westerly granite, maximum increases in penetration of 155 and 165 pct and bit life of 109 and 136 pct were obtained at or near zpc aluminum chloride concentrations of quartz (7.3 X 10-7 mol/l alcl3) and alkali feldspars (1.1 to 1.5 X 10-6 mol/l alcl3), respectively. This simultaneous increase in penetration per time together with extended bit life should result in increased productivity and reduced bit costs.