We computed zonal geopotential coefficients from average seasonal variations in global air and water mass distribution. These coefficients are used to predict the seasonal variations of LAGEOS' and Starlette's orbital node, Δ&OHgr;, and the seasonal ΔJ3 for Starlette. A comparison of these predictions with the observed values indicates that air pressure and, to a lesser extent, water storage may be responsible for a large portion of the currently unmodeled variation in the earth's gravity field. ¿ American Geophysical Union 1987