Metal forming processes typically involve changes of strain paths, which are accompanied by transients in softening or hardening behaviour. The physical cause of these transients in stress-strain responses can be attributed to the evolution of the underlying microstructural details. A crystal plasticity based model is presented to capture the complex hardening/softening transients observed in deformation of b.c.c. polycrystals at low homologous temperatures. For each crystallite, the microstructure, i.e. the cell block boundaries and cell structure, is modelled with three dislocation densities. The cell block boundaries are treated as geometrical obstacles to slip on non-coplanar slip systems. This model is implemented in a Full Constraints Taylor model to obtain the response of a polycrystal from the response of the constituent single crystals. It was found that several important features observed in the experimental stress-strain curves of b.c.c. polycrystals during complex strain paths could be reproduced. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. An rights reserved.