When horizontal stirred media mills are used to grind highly concentrated slurries with a large throughput the grinding media tend to be pushed towards the product discharge. This displacement may lead to hydraulic packing which has, in turn, negative consequences for the grinding process like ineffective energy utilisation and increased wear. The IsaMill (TM) with its internal classifier keeps the grinding media away from the discharge. This technology enabled the fine grinding of minerals in an open circuit. Since the classifier implies a current counteracting the drag forces due to the product flow, it gets more complex to understand the axial distribution of the grinding media. In order to measure the filling ratio along the M4 IsaMill (TM) a radiometric densitometer with a linear bearing was installed. This method enables the determination of the spatial filling ratio by correlating it with the attenuation of gamma radiation of a Cs-137 nuclide. Several tests have been carried out in water as a model fluid to introduce this novel technology and to characterize the general behavior of the IsaMill (TM). Future experiments will have to identify the impact of the process parameters on the axial grinding media distribution by using different Newtonian and non-Newtonian model fluids.