In noncovariant, Hamiltonian, formulation of gauge vector actions, the
conditions for invariance under electric/magnetic duality transformat
ions and their generator are transparent. Instead, satisfying Lorentz
invariance - fulfilling the Dirac-Schwinger stress tensor commutators
- becomes a complicated differential constraint on the Hamiltonian. Th
e combined requirements are equivalent to the usual Lagrangian form of
the duality invariance condition. (C) 1998 Elsevier Science B.V.