The effect of a finite mean free path on the expansion stage of central collisions between heavy nuclei is studied in a nonrelativistic fluid dynamical approach. A gas of point particles with localized interactions is used. Kinetic theory then provides the thermal conductivity and viscosity coefficients in terms of the nucleon-nucleon cross section. About 10% additional entropy is generated during the expansion stage of central collisions at a beam energy of 800 MeV/A due to viscous heating of the system. Surprisingly there is only a weak dependence on the mass of the colliding nuclei. These results are in substantial agreement with recent intranuclear cascade calculations, although at present the fluid dynamical results are slightly ambiguous due to an uncertainty in the breakup criterion. NUCLEAR REACTIONS Expansion stage of central collisions between medium energy nuclei, imperfect fluid dynamics, entropy generation, mass dependence, breakup criteria, tentative comparison with intranuclear cascade.