move relative to the dissolution medium 12 this differs markedly from prior art dissolution testing apparatus particularly of the paddle type which are designed to cause movement of the dissolution medium 12 relative to the vessel 4 and do not attempt to directly control interaction between the dissolution medium and the pharmaceutical product under test it will also be seen that the high turbidity low bulk movement fluid regime coupled with particle dispersion due to the sweeping action of the brush body 20 is a far closer approximation of the conditions within the gi tract it is therefore expected that drug dissolution test conduced in accordance with the present invention will show a far superior degree of correlation with bio studies of the same pharmaceutical products in practice selection of rotation speed is a balance between the desire to generate a consistently repeatable flow regime while preventing mechanical disruption of the pharmaceutical product by the stirring element 16 the intent is to ensure that disintegration and dissolution of the pharmaceutical product is caused by interaction with the dissolution medium 12 rather than by mechanical effects e g crushing whipping or shearing of the stirring element 16 the rotation speeds produced by conventional paddle type testing systems e g between 50 and 100 rpm will normally satisfy this requirement although the optimum speed will likely.