The influence of the strain rate on the mechanical properties of nanocrystalline Mg-23.5Ni ribbons has been determined between 175 and 250degreesC. The microstructure of the ribbon in the as-solidified condition is amorphous and crystallization takes place in two stages and, the transition between these crystallization processes occurs between 175-250degreesC. At the start of the tensile tests the microstructure of the ribbon varies between nanocrystalline at 175degreesC and a mixed nanocrystalline plus eutectic structure as the temperature increases, being. the eutectic structure: that thermodynamically stable after the second crystallization process. The material exhibits at 175degreesC the usual dependence of the stress with the strain rate, i.e. the higher the strain rate the higher the stress. At 200 and 250degreesC, the highest stress values have been attained at the lower strain rates. This anomalous mechanical behaviour is related to changes in the grain size of nanocrystals, volume fraction of the distinct phases and/or in the volume fraction of eutectic regions within the nanocrystalline matrix. At high strain rates deformation occurs practically with a stable microstructure while at low strain rates substantial changes in the microstructure takes place during the tensile test.