Viscometric and filtration properties have been studied at 80Β°F. and at elevated temperature for a saturated gypsum mud and a 20,000 ppm sodium chloride mud, treated with an acrylate-acrylamide copolymer (0.62 mole fraction sodium acrylate) and with sodium carboxymethyl cellulose (CMC). In the muds containing CMC and also for the acrylic polymer in the sodium system, the observed plastic viscosity decrease (at 175Β°F.) and the API filter loss increase (at 200Β°F.) were about what one would predict from the change in the viscosity of water with temperature. In contrast, the polyacrylate-treated gypsum mud showed a viscosity increase, rather than a decrease, a t the elevated temperature, and the APT. filter loss increased to a level substantially greater than would have been predicted on the basis of the change in the viscosity of water. This anomalous behavior of the acrylate-treated system is interpreted as evidence of polymer-calcium ion-clay interaction. Data are presented to illustrate the extent of degradation Gf the acrylic polymer and CMC in a gypsum mud when heated for 16 hr. at 350Β°F. under a nitrogen atmosphere. The room temperature filter loss of the acrylate-treated mud was scarcely changed after this treatment, while the filter loss of the CMC-treated system was comparable to that of the blank mud. These results show that, from an operational point of view, CMC was completely degraded. Despite the apparent stability of the acrylic polymer, its usefulness is severely restricted by the high visc'osities and filtration rates observed at elevated temperatures in the presence of calcium ions.