The success of the drilling operations mainly depends on rheological parameters of fluids (muds), the choice of an adequate mud to the specific particularities of a crossed formations and to a geological conditions permits therefore, not only to increase the efficiency of drilling apparatus, but also to avoid the damage (plugging) of producer layers allowing a better productivity of a drilled well. The aim of this first part of this work is to study the influence of the pressure and the temperature on rheological behavior of drilling fluids used to drill the reservoir.