Strips of 99.95 at.% Mo polycrystals annealed at 700°C as well as the ones annealed and then aged for 6 months at room temperature were deformed in tension at various strain-rates in the range 2.1 3 10 24 to 4.2 3 10 23 s 21 till fracture. It is found that natural aging of the annealed specimens for 6 months leads to 20-30% reduction in the yield stress (YS), 18-22% reduction in the ultimate tensile strength (UTS), and 72-76% reduction in the ductility, i.e. the tensile strain e max corresponding to UTS, depending on the value of _ e in the tensile strain-rate range referred to. Data analysis in terms of the kink-pair nucleation model of flow stress shows that the reduction in YS of the aged Mo specimens is a consequence of lowering of the Peierls energy per interatomic spacing along the length of screw-dislocation segments trapped in the Peierls valleys on the migration of point defects to the dislocation cores during the course of natural aging. The reduction in UTS and e max is attributed to the variation in the relative contribution of the processes of dislocation multiplication and annihilation together with the reduction in the Peierls stress of the metal.