This article discusses the generalized duality transformation in elect
romagnetics. The duality transformation affects material parameters in
the bi-anisotropic constitutive relations. It will be shown that the
two magnetoelectric phenomena, chirality and nonreciprocity, behave di
fferently in the duality transformation. The analysis of the duality t
ransformation is shown to be relevant to the question about the existe
nce of non-reciprocal. Tellegen media.