The nitrogen acceptor in zinc-blende Zn1-xMgxSe alloys has been investigated using optical as well as DLTS spectroscopy. It is found that Mg incorporation in ZnSe:N leads to the formation of an AX-centre, consisting of a shallow quasi-effective-mass state and a deeper strongly relaxed stable state. The activation energies of these two levels were determined as well as the influence of the local surroundings of the impurity.