The overall goal of this study was to determine the role of Rac1 in POSH/MLK/JNK signaling and delayed neuronal cell death following cerebral ischemia. Temporal studies revealed that Rac1 GTPase activation was significantly elevated in hippocampus CA1 at 10 min to 72 h after cerebral ischemia reperfusion, with peak levels 30 min to 6 h after reperfusion. Total Rac1 protein levels were not significantly changed following cerebral ischemia. Rac1 has been shown to interact with POSH (plenty of SH3s), a scaffold protein that binds to and regulates MLK3 and JNK activation. Co-immunoprecipitation (Co-IP) studies revealed that POSH-Rac1-MLK3 complex formation displayed a significant and prolonged elevation after reperfusion, with a correlative increase in phosphorylation/activation of MLK3 as compared to sham controls. Intracerebroventricular administration of Rac1 antisense oligonucleotides (AS-ODNs) significantly attenuated Rac1 levels and Rac1 activation at 30 min after reperfusion, with a correlated significant attenuation of POSH-MLK3-Rac1 complex formation and MLK3 activation in hippocampus CA1. Infusion of Rac1 AS-ODNs also significantly attenuated post-ischemic activation of JNK, downstream of MLK3, and strongly protected the hippocampus CA1 from ischemic damage. Missense oligos had no effect on any of the parameters measured. The Rac1 AS-ODNs results were further confirmed by administration of a Rac1 inhibitor (NSC23766), which markedly attenuated activation of Rac1 and JNK, and significantly attenuated apoptotic delayed neuronal cell death following cerebral ischemia. As a whole, these studies demonstrate an important role for Rac1 in activation of the prodeath MLK3-JNK kinase signaling pathway and delayed neuronal cell death following cerebral ischemia.