Several zero-phonon lines are seen at 4.2 °K in the absorption spectra of single crystals of magnesium oxide after irradiation with high energy neutrons. The polarization spectra of two of these lines at 27 650 and 24 200 cm^-1 have been examined under uniaxial stresses of up to 30 kg mm^-2 applied along the crystal directions [100], [111] and [110]. Both lines have the same splitting patterns and are associated with centres having [110] orientation and orthorhombic symmetry.