Experience in developing and circulating HDR reservoirs at Rosemanowes in Cornwall, UK, and more recently at Soultz-Sous-Forets in Alsace, France, suggests that the natural fracture system controls fluid flow and that artificial fractures are relatively unimportant. Two simple models, based on standard analytic solutions for the mechanical behaviour of cracks combined with a simple representation of the frictional properties of fracture surfaces, have been developed. The models describe the stimulation and circulation of HDR reservoirs in naturally fractured basement. In these models a connected fracture network is assumed, but the details of the spatial relationships between fractures within a conceptually ellipsoidal stimulated zone are ignored. On the other hand the models take close account of the size and orientation distributions of the natural fracture sets when calculating changes in fracture volume and reservoir impedance. The models provide estimates of the rock volume to fluid volume ratios required for stimulations, the injection flow rate for differing injection pressures, the fluid flux/fracture area partitioning, and hence thermal drawdown behaviour. The two models have been applied to field data from the Rosemanowes HDR research site gathered during circulation from 1985 to 1992. The first model enables the calculation of adequate stimulation pressures and fluid volumes; the second, reservoir injectivity curves and fluid flux/fracture surface area distributions within the reservoir.