Abstract: A drilling fluid containing water, a density-increasing agent, and a carbohydrate additive that is subject to deleterious attack in the drilling fluid to which is added a system that will produce in situ in the drilling fluid chlorine dioxide (ClO.sub.2) for alleviating deleterious attack on the carbohydrate additive. The system producing the chlorine dioxide must produce an initial concentration which will reduce the viable organism count and maintain a residual concentration over a protracted interval of time so as to maintain the low bacterial level, maintain the pH of the drilling fluid in the range of 6.0-8.0, and otherwise not interfere with the rheological and beneficial effects of the drilling fluid.