The mechanical and microstructural characteristics of high purity nickel processed by electrodeposition tested in compression up to a dynamic strain rate of 1.5 x 105 s-1 using a direct impact Hopkinson pressure bar technique have been analyzed. The nickel exhibits thermal activated strain rate sensitivity up to about 103 s-1. At higher rates, a sharp increase of the strength is observed related to dislocation drag effects known as the viscous regime. This strain rate dependence is best reproduced through a modified Johnson-Cook constitutive model uncoupling the strain rate dependence and the temperature dependence. Microstructure analyses reveal an increase of the average grain size with the strain rate increasing up to about 1 x 104 s-1 and are related to the reduction of amount of twins. The twin structure generated in the early shock stage of the loading evolves through dislocation interactions during the plastic deformation occurring under unshocked conditions. At a higher strain rate of 1.5 x 10 5 s-1, the grain size is decreasing to the value similar to initial state. At this strain rate, high dislocation density along with the temperature increase due to plastic deformation might favour dynamic recrystallization thus resulting in a decrease of the grain size.