The rheological behaviour of clay-polymer systems that are currently used in the formulation of drilling fluids was studied. A specific experimental procedure was used to account for thixotropic effects and obtain, thus, reproducible results. In this way it was possible to determine intrinsic properties in the whole range, from solid behaviour below the yield stress to liquid behaviour at very high shear. These systems are shown to be characterised by very long time scales of structure recovery, by the existence of a minimum shear stress in the steady state flow curve and by important stress overshoots in transient flows. These effects can be completely masked, ii standard testing procedures like the ones specified by the norms of the drilling industry are used. A recent structural constitutive equation that related thixotropic and viscoelastic properties to the formation and break-up of flocs was able to reproduce all the observed phenomena. Its ability to describe structural changes under shear or at rest as well as the associated stress growth makes it an attractive tool to use in order to improve hydraulic predictions in drilling.