The high-temperature behaviour of the AS21X magnesium alloy was tested by means of tensile and compression constant strain rate and creep tests in the temperature range between 70 and 210degreesC. Strain rate vs. stress data, representing the maximum resistance condition obtained by constant load and constant strain rate tensile data, lay on the same curves. The strain rate dependence on stress between 70 and 180degreesC was described by a conventional power law with stress exponent close to 13 and an activation energy equivalent to that for self-diffusion in Mg. Compression tests indicated that in this mode of deformation maximum resistance was slightly greater than in tension.