Rac proteins constitute a subgroup of the Rho family of small GTPases and include Rac1, which is expressed ubiquitously, and Rac2, a highly homologous protein only expressed in myelo-monocytic and lymphoid cell lineages. In fibroblasts, Rac1 plays a crucial role in control of actin cytoskeleton organisation, cell growth and Ras-induced transformation. In phagocytes, Rac1 and Rac2 regulate a specific enzymatic complex, NADPH oxidase. These multiple functions have been ascribed to Rac proteins only on the basis of cell culture and in vitro biochemical studies. To examine the role of Rac2 in vivo in a T cell lineage, we have expressed either wild-type or constitutively-activated forms of human Rac2 (Rac2V12 and Rac2L61) in transgenic mice under control of the thymus specific lck proximal promoter. We report here a striking atrophy of the thymus in mice expressing even low levels of either of the activated mutants of Rac2, while expression of Rac2wt has no effect. This phenotype is correlated with a marked decrease in the number of double positive (CD4+ CD8+) and single positive (CD4+ CD8- and CD8+ CD4-) thymocytes. Cellular and molecular analyses demonstrate that this defect is due to an increase in apoptosis among thymocytes. As Rac2 is normally expressed in thymocytes and activated T cells, we propose that Rac2 dependent pathways could play an important role in control of growth and death of T cells.