Line profiles and shifts of the Mg II 4481? line (32D-42F) were measured for a range of electron number densities from 7×1016 to 3.1×1017 clectrons/c.c. An oscillatory condenser discharge was used as a spectroscopic source. Electron-number densities were determined from measured widths of the Hβ line after addition of water vapour to the source. Good fit of the experimental contour of Hβ with the theoretical contour based on Griem-Kolb-Shen's calculation was observed. Experimental shifts toward the violet were measured to be proportion to the square of the normal microfield. An empirical quadratic Stark effect constant C4= 5.0×10-14cm4/sec was determined from these data. The widths of the line profile bear a similar relation with the fields. Ratios of the width to shift, r/△, were found to remain constant in the range of experimental conditions studied. The value is r/△ = 10.4±0.6. Theoretical calculation of the shift of the upper term 42F due to the quadratic Stark effect perturbation of 42D term allows one to explain the quadratic dependence on field, but the values of shift are smaller in comparison with experimental data, indicating an additional contribution by electrons. The widths of the line have been compared with the theoretical formula of Lindholm and Griem et al's approximate formula. Simple estimate shows a better agreement with the latter.