This paper describes part of recent studies (Briscoe, et al., Proc. 11th Inter. Congs. on Rheology, 588–590, Aug. 1992, Brussels; Briscoe, et al., The properties of drilling fluids at high pressures and high temperatures', submitted to Proc. Roy. Soc.,1992) of the rheology of concentrated clay dispersions (weight fraction from 7%–12%) under the conditions which commonly occur during oil-well drilling operations. The experimental data, which are reported in terms of the Bingham yield stress and the plastic viscosity, show that the application of high pressures modifies the Bingham characteristic parameters in a way which is both temperature and gelation-time (or shear history) dependent. The extent of this shearhistory effect may be described in terms of a “time-dependent gel strength”. The experimental results obtained with controlled shear-history measurements, using a high pressure and high temperature rolling-ball rheometer, support an extended induced volume change model, which presumes an equilibrium state for the system. The data also indicate that long time constants are associated with the gelation process at ambient temperatures of ca 20 °C. The time constants for the gelation process decrease as the temperature is increased indicating that the gelation is a thermally activated process.