The influence of grain size on the deformation of extruded Mg-3Al-1Zn tested in tension at temperatures between room temperature and 300degreesC is investigated. The results enable estimation of the deformation conditions for the transition from slip to twinning dominated flow and for the initiation and completion of dynamic recrystallization. A map illustrating these critical parameters is constructed and it is shown that the operating conditions of the common wrought processes straddle key transitions in microstructure behaviour.