The formulation of a suitable drilling fluid is necessary for the successful drilling of an oil and gas well. The rheological properties of drilling fluids vary with changes in conditions such as time and temperature. This makes it necessary to study the rheology of drilling fluids and how it is affected by these changes. As part of this work, experiments were carried out to investigate the density, rheological properties – plastic viscosity, yield point and gel strength and pH of bentonite and sepiolite water-based drilling fluid at different temperature and ageing conditions. In addition, the effect of xanthan gum commonly used as an additive in drilling fluid formulation was also investigated on the rheological properties of these muds. Results obtained indicated that sepiolite water-based drilling fluid offers better plastic viscosity and yield point as compared to bentonite water-based drilling fluids. It was also found that the plastic viscosity and yield point of sepiolite, bentonite and treated bentonite muds decreased with increase in ageing time and temperature while the gel strength increased with ageing time but also decreased with increase in temperature. Results from this work also indicated that plastic viscosity, yield point and gel strength increased as the concentration of xanthan gum increased, all of which decreased with increase in temperature. The beauty of these results is that it will enable mud engineers to appropriately understand the how these mud properties vary downhole where temperatures are relatively higher and how they vary over time.