Results of measuring nitrate (NO^-₃ ) concentrations in the upper 3-m-deep snow layer at Vostok station (Antarctica), which corresponds to the snow accumulation for the last 50 years, are presented. It has been found out that the nitrate content of snow considerably decreases with depth. The Antarctic and Greenland data series have been compared. The possibility of isolating a factor (common for both Earth's hemispheres) affecting the amount of nitrates in ice accumulations is discussed.