We report results of a molecular dynamics simulation of an ''isotope'' mixture of polymer chains, which are represented by a standard bead-spring model, and whose two species differ only by their monomer masses. Detailed analysis of the Rouse modes shows that for sufficiently short (non-entangled) chains this system can be well described by the Rouse model. Each species is described by its individual monomeric friction coefficient, whose dependence on both mass ratio as well as mixing ratio is studied. The main effect of mixing is an acceleration of the slower chains and a slowdown of the faster ones, while both species remain dynamically different. Some microscopic insight into the mechanism is obtained by studying the short-time behavior of the monomeric velocity autocorrelation function. Studies in the slightly entangled regime (chain length up to N = 150, where the typical entanglement chain length is N-e approximate to 35) seem to further corroborate the hypothesis that the ''tube diameter'' of the reptation model is a quantity which results mainly from the static configurations, i.e., is an equilibrium thermal average. The usefulness of recently suggested analysis methods in this regime is briefly discussed. (C) 1997 American Institute of Physics. [S0021-9606(97)52141-6].