The interseries interaction of the Mg 3pnd J=3 autoionizing Rydberg series has been studied using multichannel quantum-defect theory. Its impact on the excitation profiles and the autoionization rates has been investigated. The behaviors of the Mg atom and the continuity of the electron's wavefunction near the ionization threshold are examined. The agreement between the calculated antoionizing spectra and the experimental ones is found to be satisfactory.