Upstream instability and wall slip of a high molecular weight polydimethylsiloxane (PDMS) have been studied in a rough stainless-steel slit die. The velocity field at a micrometric scale close to a wall is determined with a fluorescence technique. A film of a mixture of PDMS and small fluorescent-labeled chains (PDMS-NBD) is deposited on the steel surface prior to the high-pressure flow. During the flow, the fluorescence of a small area in the middle of the surface is excited with an argon laser line and measured with a phototube. The signal decays towards an asymptotic low value, due to stray light and thermal current. The slip velocity is inferred from a comparison between the experimental decrease and a theoretical model, which takes diffusion effects into account. It is shown that the upstream instability induces a transverse oscillating velocity in the channel. A precise determination of the residual fluorescence after the wall slip, with a known diffusion coefficient and surface roughness, shows that the wall slip occurs in a plane localized at less than 1 mu m from the ridges of the surface roughness.