A microduplex stainless steel (IN744) was investigated to understand the source of variation in the parameters of the constitutive relation for superplastic deformation. In the early stage of deformation strain rate sensitivity, grain size exponent and activation energy depend on strain due to microstructural instability. After suitable prestraining such variations are sufficiently reduced and then the experimental data conform to the steady state which is an important criterion in the theoretical models. The Q obtained after suitable pre-straining of the specimens also compares with the activation energy for grain boundary diffusion in contrast to the activation energy for lattice diffusion as reported in the earlier investigations on this material. Refs.