I will review a `hidden' U(1) symmetry of the low-energy effective action on D3-branes. This U(1) acts as electromagnetic (EM) duality on the vector part of the action. I will demonstrate how the EM duality in the 4d D3-brane action is equivalent to an ordinary U(1) symmetry of the 3d M2-brane action. The oxidation of the symmetry from 3d to 4d is quite non-trivial. I will comment on a different proposed scenario for such symmetry oxidation as well as on potential implications for perturbative studies of supergravity.