A mechanism of chain slippage across the entanglement link is adopted to describe the early stage behavior in the stress relaxation of concentrated polymer systems. The chain slippage in the postulated mechanism occurs due to the equilibration of natural monomer (Rouse bead) densities between the primitive segmental units. It is shown that the processXintroduced by Lin can be confirmed theoretically and that it partially contributes to the reduction of stress discrepancy between the processesAandBof Doi.