Using molecular dynamics simulations we investigate the plastic deformation of nanocrystalline molybdenum with a grain size of 12 nm at high strain rates. The simulations are performed with an interatomic potential which is obtained through matching of atomic forces to a database generated with density-functional calculations. The simulations show the plastic deformation to involve both grain boundary processes and dislocation migration which in some cases lead to twin boundary formation. A large component of the strain is accommodated through the formation of cracks in the grain boundaries. This behavior is very different from what has been seen earlier in simulations of fee metals where grain boundary sliding is the dominant mechanism for very small grain sizes. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.