Abstract: A bore hole hydraulics simulator for drilling fluid has a pump for flowing the drilling fluid along a permeable partition which divides a first volume containing the drilling fluid from a second volume adapted to collect filtrate passing through the permeable partition. The first volume is a substantially cylindrical passageway to simulate a bore hole, having within it a rod-like member simulating a drill string. The partition is a sleeve-like insert located adjacent the rod-like member. The second volume is a chamber surrounding the insert. The rod-like member includes a device for measuring the thickness of a cake build-up on the sleeve-like insert within the passageway. The output of the pump can be adjusted in response to measurements of cake thickness, in order to maintain a substantially constant shear rate past the cake.