In the present work, starch-grafted polyacrylamide copolymers were prepared and their rheological properties, either in water or in water-based muds, were investigated. The advantages in using starches as substrates to prepare additives which are suited for improving stability and the rheological properties of water-based muds, lie in their low cost, and their lower biodegradability than native starches. Thus, various copolymer series were prepared by free-radically grafting acrylamide (AM) onto starch using ceric ammonium sulphate (CAS) as initiator. It is shown that the intrinsic viscosity and the molecular weight of the grafted starches are controlled by the initiator and monomer concentrations. Furthermore, to predict the behaviour of the samples under oilwell conditions, the aqueous solution properties of the copolymers such as water solubility, viscosity, shear rate, were determined as function of temperature, salt concentration and type, and aging time. The knowledge of these properties is a prerequisite for the use of the copolymers in drilling fluids. The data indicate that starchgrafted polyacrylamide copolymers as compared to the non-modified starch, behave as shear-thinning, are salt resistant, and their rheological properties are stable with time. The grafted starches as prepared above were also added to water-based mud and the rheological properties of the resulting muds were determined under oil-well conditions. Grafted starches, having high AM contents, are more efficient in decreasing the filtrate volume, and increasing the plastic viscosity of the muds, when compared to PAC-L, a modified cellulosic polymer used in the filtration control of most water-based muds. Such improvements in the rheological properties of the muds were found to result from the behaviour and/or properties of the grafted starches in water.