Oil and synthetic-based drilling fluids are the drilling muds of choice in high pressure and high temperature wells. The hot drilling fluid is returned to the surface and is exposed to air in the mud tanks. Since the base liquids of these drilling fluid systems are expensive, they are used in many wells before disposal. The long exposure of the oil to air can result in a substantial oxidation of the oil; especially in hot reservoirs in summer temperatures. To simulate the oxidation of these drilling fluids, an experimental study was conducted in which 20 oil-based and synthetic-based drilling fluid systems which are used in Western Canada, were examined. The drilling fluid systems were aged in stationary reactors in the presence of one charge of air for 52 hours at 150 degrees C and 13.5 MPa initial pressure. While rapid heat releases or explosions in the synthetic-based drilling fluids were observed, oil-based systems were safe under the conditions tested. The rheological properties of the drilling fluid samples after aging were compared with the original (before aging) samples. The results at an ultra low shear rate range of 0.0 - 0.2 s(-1) and a high shear rate range of 0 - 4,000 s(-1) are presented. It is shown that the aged drilling fluid systems exhibited a tremendous increase in viscosity at ultra-low shear rates. Furthermore, relatively low increases in viscosity at medium and high shear rates were observed.