Natural ageing of 99.95 at.% Mo polycrystals annealed at 700 degrees C influences its mechanical properties to a considerable extent. It is found that natural ageing for 6 months reduces the ductility by about 70% whereas yield stress and ultimate tensile strength are decreased by about 20 and 18% respectively. The stress-relaxation rate measured at a given initial stress level at which deformation is interrupted to observe stress relaxation at constant strain, is faster in aged specimens compared with that in unaged ones. Similarly, the strain-rate sensitivity of flow stress for a given stress level is also increased on natural ageing. The rate process of stress relaxation and of plastic flow at rather high strains corresponding to the parabolic stage of work hardening appears to be vacancy migration.