Rho family GTPase Cdc42 is known to regulate polarity and growth in lower eukaryotes, but its physiologic function in mammals has yet to be determined. Here we have disrupted cdc42gap, a ubiquitously expressed negative regulator of Cdc42, in mice. Cdc42GAP^-/- embryonic fibroblasts and various organs displayed significantly elevated Cdc42 activity. The embryonic and neonatal homozygous mice were reduced in size by ≈25-40% and suffered severe growth retardation. Major organs from Cdc42GAP^-/- mice were proportionally smaller because of decreased cell number. Basal apoptosis was increased in Cdc42GAP^-/- cells and tissues, and this was attributed to altered c-Jun N-terminal kinase apoptotic signals. These results reveal a role of Cdc42GAP in mammalian perinatal growth and implicate the c-Jun N-terminal kinase-mediated apoptosis machinery as a Cdc42 effector pathway in vivo.