Direct numerical simulations of transitional and turbulent flows of purely viscous thixotropic liquids in stirred tanks have been performed. The simple thixotropy model used is based on the notion of a network in the liquid with an integrity that builds up with finite rate under quiescent conditions, and breaks down under liquid deformation. We solve a transport equation for the network integrity which is two-way coupled to the lattice-Boltzmann-based flow solver. The liquid's time scale characterized by the dimensionless Deborah number shows a profound impact on the level of mobilization and the flow patterns in the mixing tanks, especially if the time scale of the liquid is of the same order as the circulation time in the tank. It is also demonstrated to what extent increasing the impeller speed improves mobilization and mixing. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2236-2247, 2010