Several myelin-derived proteins have been identified as components of the CNS myelin that prevents axonal regeneration in the adult vertebrate CNS. Activation of RhoA has been shown to be an essential part of the signaling mechanism of these proteins. Here we report an additional signal, which determines whether these proteins promote or inhibit axon outgrowth. Myelin-associated glycoprotein (MAG) and Nogo trigger the intracellular elevation of Ca2+ as well as the activation of PKC, presumably mediated by G(i)/G. Neurite outgrowth inhibition and growth cone collapse by MAG or Nogo can be converted to neurite extension and growth cone spreading by inhibiting conventional PKC, but not by inhibiting inositol 1,4,5-triphosphate (IP3). Conversely, neurite growth of immature neurons promoted by MAG is abolished by inhibiting IP3. Activation of RhoA is independent of PKC. Thus, a balance between PKC and IP3 is important for bidirectional regulation of axon regeneration by the myelin-derived proteins.