The mechanisms that underlie axon formation are still poorly understood. GSK3 has been recently implicated in establishing the axon and in its elongation. We have used four different GSK3 inhibitors to determine the role of GSK3 activity in hippocampal neurons at different periods of time. Inhibition of GSK3 activity impairs axon formation. The "critical period" of this activity of GSK3 is at least the first 24h since afterwards the inhibition of GSK3 does not compromise the process of elongation, although it exacerbates axon branching. Moreover, interference RNAs impeding the expression of the GSK3 alpha or beta isoforms in hippocampal neurons prevents an axon from forming.