The myotonic dystrophy kinase-related Cdc42-binding kinase (MRCKalpha) has been implicated in the morphological activities of Cdc42 in nonneural cells. Both MRCKalpha and the kinase-related Rho-binding kinase (ROKalpha) are involved in nonmuscle myosin light-chain phosphorylation and associated actin cytoskeleton reorganization. We now show that in PC12 cells, overexpression of the kinase domain of MRCKalpha and ROKalpha resulted in retraction of neurites formed on nerve growth factor (NGF) treatment, as observed with RhoA. However, introduction of kinase-dead MRCKalpha did not result in NGF-independent neurite outgrowth as observed with dominant negative kinase-dead ROKalpha or the Rho inhibitor C3. Neurite outgrowth induced by NGF or kinase-dead ROKalpha was inhibited by dominant negative Cdc42(N17), Rac1(N17), and the Src homology 3 domain of c-Crk, indicating the participation of common downstream components. Neurite outgrowth induced by either agent was blocked by kinase-dead MRCKalpha lacking the p21-binding domain or by a minimal C-terminal regulatory region consisting of the cysteine-rich domain/pleckstrin homology domain plus a region with homology to citron. The latter region alone was an effective blocker of NGF-induced outgrowth. These results suggest that although ROKalpha is involved in neurite retraction promoted by RhoA, the related MRCKalpha is conversely involved in neurite outgrowth promoted by Cdc42 and Rac.