The effects of temperature and alloying elements on deformation in the high-strain-rate regime were investigated by testing fine-grained magnesium alloys with an average grain size of 2 ∼ 3 μm by a nanoindentation technique. The dynamic hardness measurements aligned well with existing quasistatic data, together spanning a wide range of strain rates, 10−3 ∼ 150/s. The high-rate hardness was influenced by various alloying elements (Al, Li, Y and Zn) to different degrees, consistent with expectations based on solid solution strengthening. Transmission electron microscopy observations of the indented region revealed no evidence for deformation twins for any alloying elements, despite the high strain-rate. The activation energy for deformation in the present alloys was found to be 85 ∼ 300 kJ/mol within the temperature range of 298 ∼ 373 K, corresponding to a dominant deformation mechanism of dislocation glide.