The experimental set-up is presented in Section 2. Some general considerations on the typical regimes and pressure drop curves that are specific to an horizontal pipe and an overview of few mod-els are presented in Section 3. The experimental results are pre-sented and discussed in Section 4. The main results that concern mono-disperse calibrated solid spheres of two different sizes and two different specific masses flowing in the horizontal pipe are presented in Section 4.1. Mixture of spheres and rough stones of arbitrary shapes are also tested in Section4.2, in order to check to what extent the results obtained for spheres may represent an actual application. Several models are proposed and compared to the experiments in Section 4.3. Conclusions and perspectives are then given in Section 5