A high rate injection rheometer was developed to investigate the flow behaviour of polybutylene terephthalate (PBT) in the very conditions of injection moulding, it consists essentially of a barrel-and-die system fixed between the platens of the injection machine. Suitable transducers and the appropriate acquisition system allow all necessary data to be obtained in order to assess the entrance pressure loss and the true viscosity function. Filled PET exhibits a significant curvature of the flow curve; the entrance pressure loss is more than doubted. However, as observed with an IR camera, the extrudate swell is totally inhibited by the fibres, but viscous heat dissipation is enhanced.