There is a substantial global redistribution of atmospheric mass associated with the mean annual cycle which results in large exchanges of mass between hemispheres. Interannual fluctuations in the distribution of mass both within each hemisphere and for the ''hemispheric'' mean over 17.5¿--90¿ latitude are examined. The long-period (1899-1980) historical sea level pressures over the northern hemisphere are analyzed and found to exhibit three modes of interannual zonal mean mass redistribution that are common to all seasons. The first is one where mass is mostly redistributed within the hemisphere, while the other two modes imply that compensation of mass occurs south of 17.5 ¿N. A similar analysis over the southern hemisphere for 1972-1983 reveals remarkably similar results. Large and significant fluctuations in the hemispheric mean sea level pressure are found. These include record anomalies during the northern spring and summer of 1979, the year of the Global Weather Experiment. Opposite anomalies in the southern hemisphere, through the constraint of conservation of mass, imply the reality of a substantial global-scale redistributions of atmospheric mass.