The rheological properties of elastomeric impression materials are important because they are major determinants in the handling characteristics and adaptation to the soft and hard tissues of the mouth. Of particular interest is the rheology of the elastomeric impression materials as supplied by the manufacture since this strongly influences the ease with which the two pastes can be mixed and placed. Additionally, the flow characteristics of the mixed impression materials are also important as these will have an important bearing on the accuracy and reproduction of surface detail which are influenced by the working time. Generally, rheological properties can be defined as the study of the flow characteristics of viscos materials such as elastomeric impression materials. In the context of impression materials used in dentistry the variation of these flow characteristics over short time intervals is extremely important in the making of clinically acceptable impressions. The purpose of these studies were to measure a variety of rheological characteristic for the unmixed elastomeric impression materials and to relate this information to clinical dental procedures. The elastomeric impression materials used in this study were polysulfides, condensation cured silicone, addition cured silicone rubbers in common clinical use and specimens of adding increasing increments of filler (SiO2, TiO2 or PbO2) to a main constituent part (Thiokol LP2, Chloridparaffin, Silicone or Vinylsilicone) of each impression paste. The viscosity of these materials were measured by using the Ishida-Giken cone and plate high shear rheometer. The changes in shear stress with time were recorded for a shear rate of 5 s-1 at 23 degrees C. The relation of shear rate to shear stress were accomplished with a range of shear rates from 0 s-1 to 20 s-1. The viscosity behavior which were the flow characteristic, apparent viscosity, the yield value and the areas of hysterisis loop, were determined from both the shear stress-time and the shear stress-shear rate curves. The results of this study of elastomer impression material pastes have shown that: 1. All impression pastes exhibited non-Newtonian viscosity characteristic, apparent viscosity decreasing with increasing shear rate. 2. Surflex F base pastes (polysulfide rubber), Flexicone injection type base paste (condensation-cured silicone rubber) and Exaflex injection type base paste (addition cured silicone rubber) were pseudoplastic behavior, and other impression pastes were virtually thixotropic behavior. 3. All impression paste had yield value, especially Surflex F accelerator pastes which had a markedly higher yield value.(ABSTRACT TRUNCATED AT 400 WORDS)