A common method to characterize polymer melts is the slit rheometry. With this technique the shear viscosity can be measured up to high shear rates. For the evaluation it is assumed that within the capillary a pure shear flow exists. This paper investigates the flow patterns within such a "black box" using laser-Doppler velocimetry (LDV) which allows the measurement of the velocity fields of molten polymer melts with high spatial and temporal resolution. Therefore, this method was applied to get a deeper insight into the flow behaviour of polymer melts into and in a slit die. The materials investigated are two polyethylenes, a linear low-density polyethylene (LLDPE) and a low-density polyethylene (LDPE). Differences between these two samples are that the LLDPE is less shear thinning than the LDPE and that the elasticity is higher for the LDPE. Furthermore, the LDPE is strain hardening, the LLDPE is not. For the LDPE a pronounced secondary flow in the corners of the flow channel was found which did not occur for the LLDPE. These vortices change their shape if characterized along the neutral direction giving rise to a distinct three-dimensionality of the flow field. In addition, it was observed that within the vortices a helical flow exists which is directed to the walls of the reservoir. Special attention is paid to the velocity distribution along the symmetry axis of the channel. It was found that the LDPE shows a velocity maximum directly behind the die entrance, which does not appear for the LLDPE under the same flow conditions. It could be shown that this velocity overshoot is due to the three-dimensionality of the flow field which was observed for the strain-hardening LDPE, but not for the LLDPE. The three-dimensionality of the flow of the LDPE is marked by a velocity component in the neutral direction (z-direction), which does not appear for the LLDPE. This additional component effects a higher volume flow along the centreline in the die entrance plane. The time dependence of the rearrangement of the flow field within the slit can be related to the relaxation times of the material. (c) 2007 Elsevier B.V. All rights reserved.