The dependence of the strain-rate sensitivity coefficient of the flow stress, S, upon temperature, strain-rate and grain size in nanocrystalline metals was analyzed quantitatively in terms of the dislocation-kinetics approach; taking account of the properties of grain boundaries as sources, sinks and barriers for moving dislocations. The interaction of moving dislocations with a dislocation forest in nanograin boundaries was shown to be responsible for the fact that the values of this coefficient in nanocrystalline face-centred cubic metals (Cu, Ni) were an order of magnitude greater than those in coarse-grained metals and for the strong dependence of the coefficient, S, upon the above factors. This dependence was caused largely by annihilation of the lattice dislocations in grain boundaries controlled by the activation energy of grain boundary diffusion. The values of the coefficient, S, in nanocrystalline body-centred cubic metals (α-Fe) were an order of magnitude lower than those in coarse-grained samples, because dislocations moved in a Peierls relief in nanograins Analysis of the Strain-Rate Sensitivity of Flow Stresses in Nanocrystalline FCC and BCC Metals. G.A.Malygin: Physics of the Solid State, 2007, 49[12], 2266-73