The high concentration of the GTP-binding protein G(o) in the neuronal growth cone suggests that G(o) activation state may after neurite outgrowth. We find that activation of pertussis toxin-sensitive G- proteins by mastoparan increases neurite outgrowth from neuroblastoma cells. To examine G(o) activation specifically, point mutations homologous to activating, oncogenic mutations in alpha i2 and alpha s were introduced into the alpha subunit of G(o). The stability of the alpha o mutants to tryptic digestion confirms that they are activated. When expressed in PC12 or N1E-115 cells, activated alpha o doubles total neurites length per cell, primarily by increasing the number of neurites per cell. The growth cones of cells expressing activated alpha o are narrower than control growth cones. Expression of wildtype alpha o or the activated alpha subunits of other G-proteins did not affect total neurite length per cell. Thus, factors that lead to activation of G(o) can modulate neurite number per cell.