Commercially pure magnesium was processed by severe plastic deformation techniques and thermo-mechanical routes to produce samples with grain sizes in the range from 1.0 to 300 A mu m. Tensile testing of a fine-grained material revealed a decrease in hardening and an increase in strain rate sensitivity with decreasing testing strain rate from 10(-2) to 10(-7) s(-1). Compression testing of fine-grained samples not only revealed similar trend, but also there was a transition from twinning to slip-dominated flow. Dynamic ultra-microhardness testing showed an increase in strain rate sensitivity with decreasing grain size. The trend of decreasing hardening rate and increasing strain rate sensitivity with decreasing strain rate was not observed in samples with a coarser grain size. Compression testing at different temperatures suggests that a creep mechanism with stress exponent of similar to 7 and activation energy close to grain boundary diffusion operates at low strain rates and moderate temperatures. A model for a new deformation mechanism is proposed based on the present experimental results and published data.