  We consider some aspects of classical S-duality transformations in first
order actions taken into account the general covariance of the Dirac algorithm
and the transformation properties of the Dirac bracket. By classical S-Duality
transformations we mean a field redefinition that interchanges the equations of
motion and its associated Bianchi identities. By working from a first order
variational principle and performing the corresponding Dirac analysis we find
that the standard electro-magnetic duality can be reformulated as a canonical
local transformation. The reduction from this phase space to the original phase
space variables coincides with the well known result about duality as a
canonical non local transformation. We have also applied our ideas to the
bosonic string. These Dualities are not canonical transformations for the Dirac
bracket and relate actions with different kinetic terms in the reduced space.
