Recent evidence suggests an early involvement of dopaminergic (DA) processes and terminals in Parkinson's disease (PD). The arborization of neurons depends on the actin cytoskeleton, which in turn is regulated by small GTPases of the Rho family, encompassing Rho, Rac and Cdc42 subfamilies. Indeed, some reports point to a role for Rac and Cdc42 signaling in the pathophysiology of inherited parkinsonisms. We thus investigated the potential therapeutic effect of the modulation of cerebral Rho GTPases in PD. Cytotoxic necrotizing factor 1 (CNF1), a 114 kDa protein toxin produced by Escherichia coli, permanently activates RhoA, Rac1 and Cdc42 in intact cells. We report that the modulation of Rho GTPases by CNF1 results in hypertrophy of DA cell processes of cultured substantia nigra neurons, including increase in length, branching and varicosity. In vivo, the treatment corrects long-standing motor and biochemical asymmetries and restores degenerated nigrostriatal DA tissue after 6-hydroxydopamine lesion. We conclude that the pharmacological modulation of Rho GTPases shows neurorestorative potential and represents a promising avenue in the treatment PD. The study also suggests that naturally occurring molecules acting on Rho GTPase signaling, such as some bacterial protein toxins, might play a role in the development of PD.