The heights of the Martian ionospheric main electron density peak observed by the Mars Global Surveyor Radio Science experiment in the northern and southern high-latitude regions (60¿--90¿) are investigated. The data sets for both hemispheres were obtained in the same narrow solar longitude interval (~134¿--146¿). The subsolar height of the main peak derived from the southern hemisphere data is 7.7 ¿ 4.3 km lower than that derived from the northern hemisphere data. The southern and northern hemispheres are in opposite seasons. The seasonal variations of the neutral atmospheric density and its scale height in the lower atmosphere are found to be the dominant reasons for the reported difference. Further analysis shows that the neutral atmosphere density at the altitude 20 km had the strongest effect.