High molecular weight (10(6)) partially hydrolysed polyacrylamide (PHPA) has been prepared and its efficacy as viscosifier in water based drilling mud has been evaluated. When PHPA is added to bentonite mud system, it causes a viscosity hump indicating encapsulating effect. The breakdown point of the hump varies with the concentration of the solid mud and also with the molecular weight of the polymer. The polymer mud system is non-Newtonian in nature. The formulated polymer mud containing PHPA as viscosifier, carboxy methyl cellulose(CMC)/poly anionic cellulose(PAC) as fluid loss reducing agent, Na salt of sulphonated asphaltene as shell stabilizer, bentonite as builder and barite as high density material were evaluated at room temperature and after aging at 80-120 degrees C. Polymer mud system can be used in reservoirs having temperature below 120 degrees C. PAC shows better efficacy as fluid loss reducing agent with PHPA polymer mud than CMC.