Analyticity of nucleon form factors allows to derive sum rules which,using space-like and time-like data as input,can give unique information about behaviors in energy regions not experimentally accessible.Taking advantage from new time-like data on proton-antiproton differential cross section and hence the possibility to separate electric and magnetic form factors also in the time-like region,we verify the consistency of the asymptotic behavior predicted by the perturbative QCD for the proton magnetic form factor.