We have investigated the effect of diffusion creep on lithospheric extension using a one-dimensional numerical model that assumes a constant force is available to drive extension. The model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength. However, such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep. We consider the evolution of strain rate and temperature at the center of an idealized pure shear rift. The strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses. Given an initial thermal condition and an assumed critical value for weakening to dominate. The force at which this condition is met we will term the critical force. Diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force. We speculate that during the initial stages of continental rifting, grain size reduction, and diffusion creep deformation mechanism may have an important effect on lithospheric strength. ¿ American Geophysical Union 1993